tests: Increase timeout for test_long_readdir

This test typically needs between 70 and 85 seconds to run, so a timeout
of 90 seconds is too small if test machines are a bit slower. This
commit changes timeout for this test from 90 to 180 seconds.

Change-Id: Ieed4abd3b1ff69867a0008b88c6b6844efb1e4a6