binderRpcTest: undo ThreadPoolLimit optimization

This test started flaking when we reduced the time it took.
It should be re-written to avoid needing to sleep so long,
but for now, restoring it so it isn't breaking.

Fixes: 272429574
Test: binderRpcTest
Change-Id: Ieef2d585f6743a9a7ac79101d5f7724cff7936c3