selftests: do not macro-expand failed assertion expressions

[ Upstream commit b708a3cc9600390ccaa2b68a88087dd265154b2b ]

I've stumbled over the current macro-expand behaviour of the test
harness:

$ gcc -Wall -xc - <<'__EOF__'
TEST(macro) {
	int status = 0;
	ASSERT_TRUE(WIFSIGNALED(status));
}
TEST_HARNESS_MAIN
__EOF__
$ ./a.out
[==========] Running 1 tests from 1 test cases.
[ RUN      ] global.macro
<stdin>:4:global.macro:Expected 0 (0) != (((signed char) (((status) & 0x7f) + 1) >> 1) > 0) (0)
global.macro: Test terminated by assertion
[     FAIL ] global.macro
[==========] 0 / 1 tests passed.
[  FAILED  ]

With this change the output of the same test looks much more
comprehensible:

[==========] Running 1 tests from 1 test cases.
[ RUN      ] global.macro
<stdin>:4:global.macro:Expected 0 (0) != WIFSIGNALED(status) (0)
global.macro: Test terminated by assertion
[     FAIL ] global.macro
[==========] 0 / 1 tests passed.
[  FAILED  ]

The issue is very similar to the bug fixed in glibc assert(3)
three years ago:
https://sourceware.org/bugzilla/show_bug.cgi?id=18604

Cc: Shuah Khan <shuah@kernel.org>
Cc: Kees Cook <keescook@chromium.org>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Will Drewry <wad@chromium.org>
Cc: linux-kselftest@vger.kernel.org
Signed-off-by: Dmitry V. Levin <ldv@altlinux.org>
Acked-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Shuah Khan <shuah@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>