SkSL Persistent cache related unit-tests are flaky.

An [example of a failing run](https://logs.chromium.org/logs/flutter/buildbucket/cr-buildbucket.appspot.com/8899957189131554976/+/steps/Host_Tests_for_host_debug_unopt/0/stdout). The test seems to be failing when attempting to remove an intermediate directory.