tests: expose $(am__dry_run) limitations

tests: expose $(am__dry_run) limitations

Currently, the internal $(am__dry_run) macro can fail in weird ways
when $(MAKEFLAGS) contains shell metacharacters.  Let's expose this
limitation in the testsuite (and fix a couple of related weaknesses
since we are at it).

* tests/make-dryrun.test: Moved ...
* tests/make-dryrun.tap: ... here, converted to TAP, and extended to
expose the described limitations.  Also ...
(am_parallel_tests): Define this so that the 'gen-testsuite-part'
script won't generate a useless wrapper script.
* test/list-of-tests.mk: Update.