[SPARK-11877] Prevent agg. fallback conf. from leaking across test suites

This patch fixes an issue where the `spark.sql.TungstenAggregate.testFallbackStartsAt` SQLConf setting was not properly reset / cleared at the end of `TungstenAggregationQueryWithControlledFallbackSuite`. This ended up causing test failures in HiveCompatibilitySuite in Maven builds by causing spilling to occur way too frequently.

This configuration leak was inadvertently introduced during test cleanup in #9618.

Author: Josh Rosen <joshrosen@databricks.com>

Closes #9857 from JoshRosen/clear-fallback-prop-in-test-teardown.