tools/test-all.sh: Fix usage info

Change-Id: I7807cb372e5b278397a37fe3e685a6a919b278e4
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/51380
Commit-Queue: James Price <jrprice@google.com>
Commit-Queue: Ben Clayton <bclayton@google.com>
Auto-Submit: James Price <jrprice@google.com>
Reviewed-by: Ben Clayton <bclayton@google.com>
diff --git a/test/test-all.sh b/test/test-all.sh
index efc0091..48073bc 100755
--- a/test/test-all.sh
+++ b/test/test-all.sh
@@ -23,7 +23,7 @@
     echo "injects the <tint>/tools directory as the second command line argument"
     echo
     echo "Usage of <tint>/tools/test-runner:"
-    SCRIPT_DIR/../tools/test-runner --help
+    "${SCRIPT_DIR}/../tools/test-runner" --help
 }
 
 TINT="$1"