These tests aren't actually libc++-specific for the most part. They test
the testsuite, so they aren't tied to any implementation. That means
anybody using the testsuite should be able to run these tests
successfully.
The most notable changes are that
- `test/libcxx/selftest/modules` moved to `test/libcxx/modules`, since
the assumptions these tests check are libc++-specific
- there is a new `lit.local.cfg` for `test/selftest`
- `selftest/dsl/dsl.sh.py` was modified since the path to the monorepo
root changed
