[SPARK-15889][STREAMING] Follow-up fix to erroneous condition in StreamTest

## What changes were proposed in this pull request?

A second form of AssertQuery now actually invokes the condition; avoids a build warning too

## How was this patch tested?

Jenkins; running StreamTest

Author: Sean Owen <sowen@cloudera.com>

Closes #14133 from srowen/SPARK-15889.2.