[SPARK-15033][SQL] fix a flaky test in CachedTableSuite

## What changes were proposed in this pull request?

This is caused by https://github.com/apache/spark/pull/12776, which removes the `synchronized` from all methods in `AccumulatorContext`.

However, a test in `CachedTableSuite` synchronize on `AccumulatorContext` and expecting no one else can change it, which is not true anymore.

This PR update that test to not require to lock on `AccumulatorContext`.

## How was this patch tested?

N/A

Author: Wenchen Fan <wenchen@databricks.com>

Closes #12811 from cloud-fan/flaky.