coverage: required but missing '.am' and '.m4' files are diagnosed

coverage: required but missing '.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 the remake rules from correctly erroring out when a
still-required file is missing.

* tests/deleted-am.test: New test.
* tests/deleted-m4.test: Likewise.
* tests/Makefile.am (TESTS): Add them.