kunit: fix missing f in f-string in run_checks.py

We're missing the `f` prefix to have python do string interpolation, so
we'd never end up printing what the actual "unexpected" error is.

Fixes: ee92ed38

 ("kunit: add run_checks.py script to validate kunit changes")
Signed-off-by: Daniel Latypov <dlatypov@google.com>
Reviewed-by: David Gow <davidgow@google.com>
Reviewed-by: Brendan Higgins <brendanhiggins@google.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>