[SQL] Fixes the race condition that may cause test failure

The removed `Future` was used to end the test case as soon as the Spark SQL CLI process exits. When the process exits prematurely, this mechanism prevents the test case to wait until timeout. But it also creates a race condition: when `foundAllExpectedAnswers.tryFailure` is called, there are chances that the last expected output line of the CLI process hasn't been caught by the main logics of the test code, thus fails the test case.

Removing this `Future` doesn't affect correctness.