Add some tests on 'cygnus' mode. * tests/clean2.test: Extend. * tests/cygnus-check-without-all.test: New test. * tests/cygnus-dependency-tracking.test: Likewise. * tests/cygnus-distclean.test: Likewise. * tests/cygnus-imply-foreign.test: Likewise. * tests/cygnus-no-dist.test: Likewise. * tests/cygnus-no-installinfo.test: Likewise. * tests/cygnus-requires-maintainer-mode.test: Likewise. * tests/Makefile.am (TESTS): Update.