run_test: introduce $TESTS variable
run_test: introduce $TESTS variable

If the $TESTS variable is set to space-separated list of test-case
names, only those will be executed, e.g.:

$ make test VERSIONS=9.5 TESTS=run_s2i_test

openshift/postgresql,sclorg/postgresql-container,bparees/postgresql,pkubatrh/postgresql-container,bparees/postgresql