selftests: kvm: Introduce the TEST_FAIL macro

Some tests/utilities use the TEST_ASSERT(false, ...) pattern to
indicate a failure and stop execution.

This change introduces the TEST_FAIL macro which is a wrap around
TEST_ASSERT(false, ...) and so provides a direct alternative for
failing a test.

Signed-off-by: Wainer dos Santos Moschetta <wainersm@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>