test: replace swallowed color assertions

[RoboX2020]

Addresses the base color test part of #115.

Replace three bare except blocks with checks that can fail:

• colorname: assert the deprecation warning and the expected red name.
• ccxyz: use actual wavelength arguments instead of the nonexistent color keyword, check two spectral chromaticities and their normalization, and cover scalar input.
• colorspace_convert: check known RGB-to-HSV values and the return conversion, rather than just allocating an RGB array.

No production code changes.

Validation on Python 3.11:

• Color tests: 12 passed, 1 skipped on OpenCV 4.12.0.88 and 5.0.0.93.
• Full OpenCV 5 suite: 959 passed, 97 skipped, 5 subtests passed.
• Each rewritten test fails under its corresponding deliberate mutation: wrong color name, zero chromaticities, or skipped conversion. Mutations were restored.

The full suite was not run on OpenCV 4. Existing unrelated unused-import/variable lint findings in this test file remain unchanged. Other #115 files remain separate.

[codacy-production]

Up to standards ✅

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