kunit: tool: readability tweaks in KernelCI json generation logic

Use a more idiomatic check that a list is non-empty (`if mylist:`) and
simplify the function body by dedenting and using a dict to map between
the kunit TestStatus enum => KernelCI json status string.

The dict hopefully makes it less likely to have bugs like commit
9a6bb30a ("kunit: tool: fix --json output for skipped tests").

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>