AlertReadFailureSurfaceTests.TheNewestFailuresFactsAreNeverABlendOfTwo fails when the writers finish before the reader's first check #4312

What happened

CI failed on #4292 at 92bcaa19, in the build job's "Run Darling tests" step, with one failure:

Darling.Tests.AlertReadFailureSurfaceTests.TheNewestFailuresFactsAreNeverABlendOfTwo: "the reader observed nothing, so its silence proves nothing" (Darling/Darling.Tests/AlertReadFailureSurfaceTests.cs, line 803).

#4292 does not touch this file or AlertReadFailureCounter.

Why it fails

The test starts five writer tasks with TaskCreationOptions.LongRunning. Then it reads on the test thread:

while (!writers.All(w => w.IsCompleted))
{
    var reading = counter.ReadFor(Key);
    observations++;
    // ...
}

A while loop checks its condition before its first pass. If all five writers finish before the test thread first checks it, the body never runs, and observations stays 0. On a busy runner, the test thread can wait for a CPU while five dedicated writer threads run 200,000 iterations each.

The comment above the assert says that the seeded bucket makes one observation certain. The seed makes the first reading complete, but it does not make the loop run.

Fix

Make the loop run at least once:

do
{
    var reading = counter.ReadFor(Key);
    observations++;
    // ...
}
while (!writers.All(w => w.IsCompleted));

The seeded bucket then makes the first observation a complete trio, as the comment says. This is a change to the test only.