CTest included when tests are disabled

[xoorath]

Hey,

When HWY_ENABLE_TESTS is OFF: CTest is still included.

This isn't a big problem but it does put a bunch of projects into my solution that I don't need.

One hacky workaround is to set CTEST_TARGETS_ADDED 1 before making highway available.

[jan-wassenberg]

Makes sense. Note that CTest defines BUILD_TESTING, but we could do this:

if(HWY_ENABLE_TESTS)
include(CTest)
if(BUILD_TESTING)#was both conditions before

...

endif() # existing
endif() # new

Would you like to send that change as a PR?