[SPARK-18031][TESTS] Fix flaky test ExecutorAllocationManagerSuite.basic functionality

## What changes were proposed in this pull request?

The failure is because in `test("basic functionality")`, it doesn't block until `ExecutorAllocationManager.manageAllocation` is called. This PR just adds StreamManualClock to allow the tests to block on expected wait time to make the test deterministic.

## How was this patch tested?

Jenkins

Author: Shixiong Zhu <shixiong@databricks.com>

Closes #16321 from zsxwing/SPARK-18031.

(cherry picked from commit ccfe60a8)
Signed-off-by: Tathagata Das <tathagata.das1565@gmail.com>