Configure Tests/CMakeTests only with BUILD_TESTING ON

Since commit e03f83f3 (ProcessorCount test: fix path to cmsysTestsCxx
executable, 2013-01-24) the directory references a target that is not
built when BUILD_TESTING is OFF.  It makes no sense to add tests without
BUILD_TESTING anyway.

While at it, use add_subdirectory instead of subdirs to add CMakeTests.
Merge topic 'configure-no-tests' into next

dd8a06b Configure Tests/CMakeTests only with BUILD_TESTING ON