test(anagram): clean up redundant tests

[ofhope]

Going through this exercise I notice a fair bit of redundant testing. They loose value making it harder to establish the expected return.

• Removed multiple tests for detects multiple anagrams
• Removed multiple tests for anagrams must use all letters exactly once
• Removed multiple test for case insensitive checking
• Clarified test descriptions

[github-actions]

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[ofhope]

There is no distinction between simple and complex anagrams. Removing this adjective.