coverage: tests AM_MISSING_PROG usage

coverage: tests AM_MISSING_PROG usage

Now that AM_MISSING_PROG is documented, we want to make sure it
works as expected when used in third-party code.

* t/am-missing-prog.sh: New test.
* t/ammissing: Rename ...
* t/am-macro-not-found.sh: ... like this, to avoid confusion (this
test has nothing to do with the 'missing' script nor with the
'AM_MISSING_PROG' macro).
* t/list-of-tests.mk: Adjust.

Signed-off-by: Stefano Lattarini <stefano.lattarini@gmail.com>