TST: add robust tests for exec_command()

First, we test both stdout and stderr.

Second, we now test both _exec_command_posix() and _exec_command(), see
numpy/distutils/exec_command.py. This reveals a bug in the implementation which
will be fixed in the next commit. For now the failing test is commented out so
that all tests still pass.
Implement separate checking for stdout and stderr

And enable a test for this that was failing before.