selftests: harness: avoid false negatives if test has no ASSERTs

Test can fail either immediately when ASSERT() failed or at the
end if one or more EXPECT() was not met. The exact return code
is decided based on the number of successful ASSERT()s.

If test has no ASSERT()s, however, the return code will be 0,
as if the test did not fail. Start counting ASSERT()s from 1.

Fixes: 369130b6 ("selftests: Enhance kselftest_harness.h to print which assert failed")
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>