Whitelist a "test" directory for unit tests

[ianfixes]

Library Specification, proposed amendment

Several unit testing systems exist for Arduino libraries; this patch creates an official "space" within a library to hold whatever files may relate to that functionality. Unlike "Extras", the test files would not become part of the bundled library.

Text adapted from #990 (comment)

• Fixes Enable unit tests - ignore a test/ directory in an Arduino library #990 (amends the spec, but does not implement the functionality)

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