Assert max uid to detect uid range issues (#275)

## Checklist before submitting

- [X] Did you read the [contributing
guide](https://github.com/G-Research/spark-dgraph-connector/blob/contributing-guidelines/CONTRIBUTING.md)?
- [ ] Did you update the docs?
- [ ] Did you write any tests to validate this change?  

## Description

Asserts the max uid of the Dgraph instance used for unit tests to flag
unexpected large uid ranges.

## Review process for approval

1. All tests and other checks must succeed.
2. At least one core contributors must review and approve.
3. If a core contributor requests changes, they must be addressed.

src/test/scala/uk/co/gresearch/spark/dgraph/DgraphTestCluster.scala

@@ -41,7 +41,10 @@ trait DgraphTestCluster extends BeforeAndAfterAll {
 
   lazy val dgraph = new DgraphCluster(alwaysStartUp = clusterAlwaysStartUp)
 
-  override protected def beforeAll(): Unit = dgraph.start()
+  override protected def beforeAll(): Unit = {
+    dgraph.start()
+    assert(dgraph.highestUid < 1024)
+  }
 
   override protected def afterAll(): Unit = dgraph.stop()
 }