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

[cloud-fan]

What changes were proposed in this pull request?

This is caused by #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

[cloud-fan]

cc @yhuai @rxin

[SparkQA]

Test build #57428 has finished for PR 12811 at commit 1a5234e.

• This patch fails Spark unit tests.
• This patch merges cleanly.
• This patch adds no public classes.

[cloud-fan]

retest this please

[SparkQA]

Test build #57437 has finished for PR 12811 at commit 1a5234e.

• This patch fails Spark unit tests.
• This patch merges cleanly.
• This patch adds no public classes.

[SparkQA]

Test build #2952 has finished for PR 12811 at commit 1a5234e.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds no public classes.

[SparkQA]

Test build #2951 has finished for PR 12811 at commit 1a5234e.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds no public classes.

[SparkQA]

Test build #2953 has finished for PR 12811 at commit 1a5234e.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds no public classes.

[rxin]

Merging in master.