tests: more faithful 'installcheck' support in few tests

tests: more faithful 'installcheck' support in few tests

* tests/test-driver-acsubst.test: Use the `$am_scriptdir'
variable instead of `$testsrcdir/../lib', to test more
faithfully under "make installcheck".
* tests/test-driver-cond.test: Likewise.
* tests/dist-auxfile.test: Likewise, and add one more use
of `fatal_' to report hard errors.
* tests/tests/dist-auxdir-many-subdirs.test: Likewise.