Add local cache unit test for long async refresh case

[paggynie]

This is a follow up on #6851 about adding unit test to avoid similar breaking change happen again. This is a generic test to test async refresh on another thread takes long enough for entry to be expired and we still expect to view the reload result without any exception.

I verified with change introduced from #6851 , the test failed with

Tests run: 9571, Failures: 0, Errors: 1, Skipped: 0, Time elapsed: 1.677 sec <<< FAILURE!
testLongAsyncRefresh(com.google.common.cache.LocalCacheTest)  Time elapsed: 0.102 sec  <<< ERROR!
java.lang.IllegalStateException: Recursive load of: test

after the change from #6851 roll back, the test passes

Running com.google.common.cache.LocalCacheTest
Tests run: 9571, Failures: 0, Errors: 0, Skipped: 0, Time elapsed: 4.962 sec

[cpovirk]

@paggynie, my internal reviewer ended up challenging me to rewrite this test in a way that avoided sleep and otherwise guaranteed that it would catch the bug, no matter what happened with scheduling. I think I managed that in d5fbcca, but if anything worries you about my changes, let me know. I still left the PR credited to you, since you reported the bug and provided the PR that I started from. But if anyone tells you there was something wrong with the commit that we merged with your name on it, you can point to my explanation here :) Sorry for taking so long on this, and sorry again for the bug.