Add missing tests for data-point-cache/lib/cache.js

[raingerber]

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Problem description:

When upgrading from Jest 21 to 23, the test coverage decreased unexpectedly. For now, <rootDir>/packages/data-point-cache/lib/cache.js has been added to coveragePathIgnorePatterns.

Suggested solution:

Add tests that cover the untested lines, and remove cache.js from coveragePathIgnorePatterns.