kunit: tool: fix --alltests flag

Alltests flag evidently stopped working when run from outside of the
root of the source tree, so fix that. Also add an additional broken
config to the broken_on_uml config.

Signed-off-by: Brendan Higgins <brendanhiggins@google.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>