Rename karma.testDirs config to something else

[insin]

Default config is to ignore dirs which follow certain common test layout conventions (top-level test(s)/, nested __tests__), but this config can be used to exclude anything from coverage, including individual files.

This would make it clearer it's the right config to use for #235

How about karma.excludeFromCoverage?

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Also, drop all mention of testFiles in the docs section about ignoring stuff as it's just telling you irrelevant implementation details.