kunit: tool: fix --json output for skipped tests

Currently, KUnit will report SKIPPED tests as having failed if one uses
--json.

Add the missing if statement to set the appropriate status ("SKIP").
See https://api.kernelci.org/schema-test-case.html

:
  "status": {
      "type": "string",
      "description": "The status of the execution of this test case",
      "enum": ["PASS", "FAIL", "SKIP", "ERROR"],
      "default": "PASS"
  },
with this, we now can properly produce all four of the statuses.

Fixes: 5acaf603 ("kunit: tool: Support skipped tests in kunit_tool")
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>