Improve comments in tests `posixsubst*.test'. * tests/posixsubst-data.test: Improve comment explaining why we try also empty match suffix. * tests/posixsubst-extradist.test: Likewise. * tests/posixsubst-ldadd.test: Likewise. * tests/posixsubst-libraries.test: Likewise. * tests/posixsubst-ltlibraries.test: Likewise. * tests/posixsubst-programs.test: Likewise. * tests/posixsubst-scripts.test: Likewise. * tests/posixsubst-sources.test: Likewise. * tests/posixsubst-tests.test: Likewise. Suggested by Ralf Wildenhues.