Assert max uid to detect uid range issues

[EnricoMi]

Checklist before submitting

• Did you read the contributing guide?
• 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.

[github-actions]

Test Results

 2 185 files  ±0   2 185 suites  ±0   43m 28s ⏱️ +43s
   485 tests ±0     485 ✅ ±0  0 💤 ±0  0 ❌ ±0 
32 711 runs  ±0  32 711 ✅ ±0  0 💤 ±0  0 ❌ ±0 

Results for commit 9a6fdca. ± Comparison against base commit 72d1349.