Use relative paths for spec tests to skip to avoid ambiguity when two tests have the same basename (#8060)

* Update SPEC_TESTS_TO_SKIP and SPEC_TESTSUITE_TESTS_TO_SKIP to use
relative paths, which allows us to distinguish between tests with the
same base name. This is useful since some future changes will fix
test/spec/testsuite/memory.wast but not
test/spec/testsuite/proposals/threads/memory.wast
* Check that the user-specified `--spec-test` paths exist. This is also
done to avoid skipping rather than erroring when the user specifies a
path relative to `test/` instead of relative to the repo root (since we
drop the test/ prefix we can't tell the difference).