tests: fix timestamp-related failures Fixes automake bug#8365. * tests/aclocal6.test: Sleep before modifying m4 files that should trigger remake rules. Remove incorrect/obsoleted comments. * tests/subdir5.test: Likewise, and extend a bit. * tests/subdir8.test: Likewise. * tests/pr8365-remake-timing.test: New xfailing test. * tests/Makefile.am (TESTS): Update. Report from Sam Steingold.