Document RUN_TESTS target under enable_testing()
I think it'd be good to document the RUN_TESTS target as a side effect of calling enable_testing().
I was having issue with why the RUN_TESTS target wasn't getting created and after poking around found on the wiki some info about it which led me to enable_testing, which mentioned calling from the root directory (I was calling it from a tests directory).
Anyways, would be good to consolidate this information in the docs. Also (maybe separate issue?), somehow warn when enable_testing() is called but RUN_TESTS target doesn't get generated.