abicompat: Fix exit code in weak mode

abicompat: Fix exit code in weak mode

It turns out the tool is almost always returning ABIDIFF_OK in weak
mode.  Oops.  Fixed thus.  Also, update the test-abicompat.cc to test
for expected exit codes to catch this kind of regressions in the
future.

	* tools/abicompat.cc (perform_compat_check_in_weak_mode): Do not
	override the status code when doing the comparison in the reverse
	direction.
	(compare_expected_against_provided_functions)
	(compare_expected_against_provided_variables): Set the status code
	close to the detected diff.  In the future, this might help us
	provide finer grained status.
	* tests/test-abicompat.cc (InOutSpec::status): Add a new data
	member to represent the expected exit code.
	(in_out_specs): Adjust the array of tests.
	(main): If the actual exit code is different from the expected
	one, then the test failed so let's report it.

Signed-off-by: Dodji Seketeli <dodji@redhat.com>