integration-tests: Fix `run-tests.sh --sources DIR` when DIR doesn't exist

We need to canonicalize the path using the realpath -m or realpath --canonicalize-missing, otherwise if the directory doesn't exist we get an error:

realpath: /tmp/foo/bar/: No such file or directory

The tests still work if this happens but the sources go into the default location because the BST_SOURCE_CACHE variable goes unset.