Add more specific tests

[IanButterworth]

Tests currently only test that there are objects detected https://github.com/r3tex/ObjectDetector.jl/blob/9b5851327d3bbb53893d93af9e9928b6fad69207/test/maintests.jl#L51

We really should do all or some of these at least:

• Test for exact numerical parity in returned array with cached result
• Test IoU with cached result
• Compare for visual regression in result image with labeled bboxes