Fix "unknown option" error message from runner

[jmid]

While switching from dune exec path/some.exe -- -v -s 123 to _build/default/path/some.exe I mistakingly passed -- which is not a valid QCheck option and was surprised by this error message:

_build/default/path/some.exe: unknown option '--'.
run qtest suite
-v --verbose enable verbose tests
--colors colored output
[...]

This PR therefore changes the second line to

run QCheck test suite