Pass $(MAKE) down to testsuite.

Pass $(MAKE) down to testsuite.

If Make is not available under the command name ‘make’, only some
other name (e.g. ‘gmake’) then the test suite’s internal invocations
of Make will all fail unless you explicitly set MAKE=<the other name>
in the environment, which is obnoxious.  Pass the value of $(MAKE)
down to the testsuite so that ‘gmake check’ Just Works.

 * tests/local.mk (run_testsuite): Append MAKE=$(MAKE).