Rename 'test' to 'check' target in Makefile.direct

Rename 'test' to 'check' target in Makefile.direct

This is to match the target name in Makefile generated by configure.

* Makefile.direct: Update comment about testing (rename "test" target
to "check").
* Makefile.direct (check): Renamed from "test".
* Makefile.direct (test): Depend on check; add comment.
* README.QUICK (INSTALLATION): Leave information only about the recent
GC version.
* README.QUICK (INSTALLATION): Replace "make test" with "make check".
* README.md (Installation and Portability): Likewise.