Fix not-quite-right Makefile for src/test/modules/test_checksums.

Fix not-quite-right Makefile for src/test/modules/test_checksums.

This neglected to set TAP_TESTS = 1, and partially compensated
for that by writing duplicative hand-made rules for check and
installcheck.  That's not really sufficient though.  The way
I noticed the error was that "make distclean" didn't clean out
the tmp_check subdirectory, and there might be other consequences.
Do it the standard way instead.