Add test case for concurrent callers throwing in AsyncAtomicFactory.GetValueAsync

[bitfaster]

When called concurrently, AsyncAtomicFactory.GetValueAsync will enforce that one thread is calling the factory method.

Let's say

• There are two threads, A and B, that both call GetValueAsync.
• Thread A starts first
• Thread B starts while thread A is running
• Thread A throws an exception
• Thread B should see the exception from Thread A

In other words, a second caller to GetValueAsync that overlaps the first should effectively queue on the same underlying task, and if the first throws, the second task should see that exception. This PR adds a test case to verify this.

By queuing on the same task like this, we mitigate lock convoys as discussed in #477.

[coveralls]

coverage: 99.235% (+0.1%) from 99.118%
when pulling c0c2a99 on users/alexpeck/testatomfailure
into 5b2d64a on main.