Fix race in flaky LoadingMapTest.values

[jkni]

There is a minor race in this test if the main thread runs collect and reaches the entry for key 3 before the executor starts running the update for key 3.

[jkni]

@jacek-lewandowski would you mind reviewing here? It looks like you are the primary author of this collection/test. I am also curious if we should remove this test and valuesStream. valuesStream isn't used outside of the test, and versions of this patch committed elsewhere do not include valuesStream.

[JeremiahDJordan]

Don't know about the removal, but the fix lgtm.