Setting WITH_TESTING=yes will cause running make test between stage and
package in the default pipeline.

This feature is off by default.

Another change that I put into this diff is

Mk/bsd.port.mk: Allow WITHOUT_FEATURE_PORTS to cancel WITH_FEATURE

I had to rewrite the loop as @pizzamig initially suggested.

Tested the feature itself on devel/zfp (that has USES=cmake:testing) and graphics/ImageMagick7 (which has just TEST_TARGET).

Tested WITH/WITHOUT stuff the following way:

make -C audio/abx -V _WITH_TESTING

make -C audio/abx -V _WITH_TESTING WITH_TESTING=yes
yes

make -C audio/abx -V _WITH_TESTING WITH_TESTING=yes WITHOUT_TESTING_PORTS=audio/abx