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.

integration-tests/run-test.sh

@@ -56,10 +56,10 @@ main () {
 				clean "$@"
 				break ;;
 			--sources)
-				export BST_SOURCE_CACHE=$(realpath "${2}")
+				export BST_SOURCE_CACHE=$(realpath -m "${2}")
 				shift 2 ;;
 			-c|--cov)
-				export BST_COVERAGE=$(realpath "${2}")
+				export BST_COVERAGE=$(realpath -m "${2}")
 				shift 2 ;;
 			-a|--arg)
 				export BST_FLAGS="${BST_FLAGS:-} $2"