coverage: undistributed '.am' and '.m4' files are diagnosed

coverage: undistributed '.am' and '.m4' files are diagnosed

The stub rules emitted to work around the "deleted header problem"
for `.m4' files (included by autoconf in e.g., configure.ac) and
for `.am' files (included by automake in e.g., Makefile.am) should
not prevent "make" from correctly complaining when such a required
file is missing from a distribution tarball.
* tests/dist-missing-am.test: New test.
* tests/dist-missing-m4.test: Likewise.
* tests/dist-missing-included-m4.test: Likewise.
* tests/Makefile.am (TESTS): Add them.

Suggestion by Ralf Wildenhues.