add mock for status.uptime unit test

[jfindlay]

What does this PR do?

Fix the intermittent stack trace from the status.uptime unit test on macOS.

Traceback (most recent call last):
  File "/testing/tests/unit/modules/status_test.py", line 45, in test_uptime
    u_time = status.uptime()
  File "/testing/salt/utils/decorators/__init__.py", line 579, in _decorate
    return self._call_function(kwargs)
  File "/testing/salt/utils/decorators/__init__.py", line 314, in _call_function
    raise error
CommandExecutionError: This platform is not supported

What issues does this PR fix or reference?

#37157. @rallytime, this change should not be included in a merge to develop if/when #37157 is merged.

Tests written?

Yes