Improve comments in tests `posixsubst*.test'.

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.