tools: Add new "test" taint to kernel-chktaint

Commit c272612c ("kunit: Taint the kernel when KUnit tests are run")
added a new taint flag for when in-kernel tests run. This commit adds
recognition of this new flag in kernel-chktaint.

With this change the correct reason will be reported if the kernel is
tainted because of a test run.
Amended Commit log: Shuah Khan <skhan@linuxfoundation.org>

Reviewed-by: David Gow <davidgow@google.com>
Signed-off-by: Joe Fradley <joefradley@google.com>
Reviewed-by: Brendan Higgins <brendanhiggins@google.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>