tests: reenable "make -C tests ..." commands

tests: reenable "make -C tests ..." commands

The README-documented way to run individual tests was invalidated
by the conversion of tests/ to non-recursive make.  Add a GNUmakefile
shim to reenable that usage.
* tests/GNUmakefile: New file, so that "make -C tests ..." works
like it did before the conversion of tests/ to non-recursive build.
Reported by Bernhard Voelker.
* Makefile.am (EXTRA_DIST): Add it.
* cfg.mk (sc_prohibit_tab_based_indentation): Also exempt any
GNUmakefile from this syntax-check.