Fix TestKit version for each driver commit (#723)

Co-authored-by: grant lodge <[email protected]>

Author: robsdedude

testkit/testkit.json

@@ -0,0 +1,6 @@
+{
+  "testkit": {
+    "uri": "https://github.com/neo4j-drivers/testkit.git",
+    "ref": "5.0"
+  }
+}