Use $(MAKE) instead of ${MAKE}, and fixup two more make invocations.

Include a new SINGLE_MAKE which can be used to invoke make but using
only a single job, and in way that avoids any warnings from make.
Use SINGLE_MAKE to execute the tests, since they're meant to be run
serially.

Also, prefer the use of $(MAKE) to avoid invoking an extra subshell
(saves some time).