tests: Fix RunCMake.CMP0052 test on OS X

Use min req 2.8.12 instead of 3.0 because the latter causes the
-DCMAKE_POLICY_DEFAULT_CMP0025=NEW option RunCMake.cmake passes on the
command line on APPLE to go unused, and that warning breaks the expected
results.  A future topic will need to convert RunCMake testing to work
with min req 3.0.
Merge topic 'dev/faster-evis' into next

c9ba7003 tests: Fix RunCMake.CMP0052 test on OS X