Add TEST_INCLUDE_FILES

Matthew Woehlkerequested to merge
matthew-woehlke/cmake:test_include_files into master

Add new directory property TEST_INCLUDE_FILES. This supersedes TEST_INCLUDE_FILE, though the latter is of course retained for compatibility.

Basically, this is a list rather than a single file. This allows the feature to be used by generic utilities without conflicting with local use.

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