Improve OverlordResourceTest to actually verify API calls

[jihoonson]

Currently OverlordResourceTest is calling Java methods directly instead of calling HTTP endpoints. This sometimes makes somethings to test hard. For example, #6679 isn't easy to test by directly calling Java methods. Testing resourceFilters is also difficult.

Probably this test can be improved using jersey-test-framework or something else similar. Also we may need to consider upgrading jersey first.

[QiuMM]

Maybe other resource test classes should be improved at the same time.

[jihoonson]

@QiuMM yes, sounds great.

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