tests: better name for a couple of tests

tests: better name for a couple of tests

Two tests checking "make -n" behaviour with the parallel-tests
harness has been introduced in the Automake repository in the past:
one of them ('parallel-tests-dryrun.test') in the maint branch, the
other one ('parallel-tests-dry-run.test') in the master branch.

Needless to say, their too-much-similar names can be a great source
of confusion now.  Moreover, the two tests have been written during
distinct refactorings, and are meant to exercise different code
paths and/or usage scenarios, so that no one of them subsumes the
other, and they should be both kept.

Thus we simply rename both of them to avoid future confusion.

* tests/parallel-tests-dryrun.test: Renamed ...
* tests/parallel-tests-dry-run-1.test: .. to this.
* tests/parallel-tests-dry-run.test: Renamed ...
* tests/parallel-tests-dry-run-2.test: .. to this.
Add cross-reference among them.
* tests/list-of-tests.mk: Adjust.