Allow nested ordering and interleaved order

[yalon]

Enhance the ordering groups to allow nesting, and create a new "interleaved" order group.

Nesting is helpful to allow for a more modular test code - you can write methods that create a group of functions that execute in some order.

group1 = EnforceSequentialOrder()
with group1:
 foo1()
 foo2()

# This will append to the same group
with group1:
 foo3()
 foo4()

# This will create a group that doesn't care about order:
group2 = UnorderdGroup()
with group2:
 bar1()
 bar2()

# Now we can do this:

with AllowInterleavedOrder():
 group1()
 group2()

# This means that the following is valid:
#1. foo1, foo2, bar1, foo3, foo4, bar2
#2. bar2, foo1, bar1, foo2, foo3, foo4
#
# This is invalid:
#1. foo2, foo1, ...
#2. missing bar
# ...
# You get the picture :)