add test_scheduler to support marble tests

[kirkshoop]

test_context implements a time-scheduler with additional test features.

test_context is used to build tests using marble sequence senders
and marble recorders

test_clock_context & test_clock are used to represent time for the tests

test_scheduler is used to queue tasks on the test_context at virtual-time points

One or more __test_sequence(s) are constructed from marble
diagrams and each schedules marbles on the test_scheduler
when connected and started

Once one or more __test_sequence(s) have been composed into an expression,
the test_context is used to record the expression results as a
set of marbles. To support the testing of infinite sequences
the recording will be requested to stop at a default of 1000ms from
start() if the expression has not completed.

The expression result marbles are then compared to an expected
set of marbles generated from a separate marble diagram

An example of usage:

TEST_CASE(
"test_scheduler - test_context marble-sequence never",
"[sequence_senders][test_scheduler]") {
test_context __test{};
auto __clock = __test.get_clock();
CHECK(test_clock::time_point{0ms} == __clock.now());
// a sequence that will produce '0' at 1ms from start()
// and then never complete
auto __sequence = __test.get_marble_sequence_from(
" -0-"_mstr);
// the set of marbles for a sequence that contains '5'
// at 1ms from start() and then is externally stopped
// after 1000ms have elapsed since start()
auto expected = get_marbles_from(__clock,
"=^-5 998ms $"_mstr);
// record an expression that is expected to turn '0' to '5'
auto actual = __test.get_marbles_from(
__sequence
| then_each([](char c){ return c+5; }));
CHECK(test_clock::time_point{1000ms} == __clock.now());
CAPTURE(__sequence.__marbles_);
CHECK(expected == actual);
}

with output:

-------------------------------------------------------------------------------
test_scheduler - test_context marble-sequence never
-------------------------------------------------------------------------------
/home/coder/stdexec/test/exec/sequence/test_test_scheduler.cpp:114
...............................................................................
/home/coder/stdexec/test/exec/sequence/test_test_scheduler.cpp:117: PASSED:
CHECK( test_clock::time_point{0ms} == __clock.now() )
with expansion:
0ms == 0ms
/home/coder/stdexec/test/exec/sequence/test_test_scheduler.cpp:123: PASSED:
CHECK( test_clock::time_point{1000ms} == __clock.now() )
with expansion:
1000ms == 1000ms
/home/coder/stdexec/test/exec/sequence/test_test_scheduler.cpp:125: PASSED:
CHECK( expected == actual )
with expansion:
{ sequence_connect()@0ms, sequence_start()@0ms, set_value('5')@1ms,
sequence_stopped()@1000ms }
==
{ sequence_connect()@0ms, sequence_start()@0ms, set_value('5')@1ms,
sequence_stopped()@1000ms }
with message:
__sequence.__marbles_ := { set_value('0')@1ms }

Marbles:

marble_t<_Clock> represents a signal for a sequence
sender and the frame at which the signal occurs/occured

a marble diagram is a string that is parsed into a
vector<marble_t<_Clock>>

Example:
this marble diagram
"--a--b--c|"
is equivalent to this set of marble_t

 marble_t{2ms, ex::set_value, 'a'},
marble_t{5ms, ex::set_value, 'b'},
marble_t{8ms, ex::set_value, 'c'},
marble_t{8ms, sequence_end}

which is displayed as

 set_value('a')@2ms, set_value('b')@5ms,
set_value('c')@8ms, sequence_end()@8ms

Diagram reference:
Time:

 ' ' indicates that 0ms has elapsed - used to line up diagrams in a visually pleasing manner
'-' indicates that 1ms has elapsed
' 0-9+(ms|s|m) '
indicates elapsed time at that point in the diagram that is equal to the specified number
of ms - milliseconds, s - seconds, m - minutes
NOTE: must have a preceding and following space to disambiguate from values
'(' begins a group of signals that all occur on the frame that the group begins on
')' ends a group of signals that all occur on the frame that the group begins on

Value:

 '0'-'9' 'a'-'z' 'A'-'Z'
indicates that a value in the sequence completes with set_value( char )
'#' indicates that a value in the sequence completes with set_error(error_code(interrupted))
'.' indicates that a value in the sequence completes with set_stopped()

Sequence:

 '=' indicates connect() on the sequence sender
'^' indicates start() on the sequence operation
'|' indicates that the sequence completes with set_value()
'$' indicates that the sequence completes with set_stopped()
'?' indicates that request_stop() was sent to the sequence from an external source

record_marbles() will record a set of marbles from the signals of the specified sequence sender

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[ericniebler]

/ok to test 84b041f

[ericniebler]

/ok to test 0fc8423

[ericniebler]

/ok to test 850a25c

[ericniebler]

/ok to test 307ead3

[ericniebler]

/ok to test ce4b544

[kirkshoop]

This is ready to try the CI again

[ericniebler]

/ok to test 0e3c0c4

[ericniebler]

/ok to test 8e54b92

[ericniebler]

/ok to test 92cf09d

[ericniebler]

/ok to test d8d63a7