[flang] Skip `Fortran STOP:` before message when NO_STOP_MESSAGE is set

In 'STOP bye bye', do not print 'Fortran STOP:` before 'bye bye' when
NO_STOP_MESSAGE environment variable is set at runtime.

Also only exit with code 1 in StopStatementText if this is an ERROR STOP.
This matches other compiler behaviors.

Move STOP related unit tests in their own test file and add new tests to
cover this change.

Differential Revision: https://reviews.llvm.org/D114152