libs.mk,build/make/Makefile: make test targets ordinary rules

libs.mk,build/make/Makefile: make test targets ordinary rules

this fixes a regression in make 4.2 and still present in 4.3 causing
double colon rules to be serialized which breaks sharding done by the
test and test-no-data-check rules. these targets only define one set of
rules so ordinary rules work unlike clean. install may be another
candidate, but that's left for a follow up.

Change-Id: I9f074eca2ad266eeca6e31aae2e9f31eec8680e0
Tested: make 3.81, 4.1, 4.2, 4.2.1, 4.3