Serialize SelfLog-sharing tests to fix intermittent CarriesMessageTemplateProperties failure

[thekim1]

Problem

I saw the pipeline fail in the build and publish stage after my last pr merged. Some tests failed in the pipeline even if they worked on local dev machine so I had a look at it.

CI failed intermittently in the Testing stage on net8.0/net9.0 (passed on net10.0/net48):

Failed Tests.Serilog.Extensions.Logging.SerilogLoggerTests.CarriesMessageTemplateProperties
  System.Exception : ... Exception System.Exception: Enricher Failed
     at Tests.Seq.Extensions.Logging.EnricherTests.<>c.<FailingEnricherIsHandled>b__1_1(...)
     at Seq.Extensions.Logging.Enricher.Enrich(...) ... caught while enriching ...

CarriesMessageTemplateProperties enables SelfLog, logs a message, asserts the captured
output is empty, and throws if not. The captured text was "Enricher Failed" — the exception
thrown by EnricherTests.FailingEnricherIsHandled.

Root cause

SelfLog is a process-global singleton (static Action<string>? _output) with no per-test
isolation. The two tests sit in different xUnit collections, which xUnit runs in parallel by
default. When FailingEnricherIsHandled triggers its failing enricher during the window where
CarriesMessageTemplateProperties has SelfLog._output pointed at its StringWriter, the
diagnostic write leaks into the other test's writer, making it throw the captured text.

It is timing-dependent (a race), which is why it surfaced on only 2 of 4 target frameworks in
this run and never reproduces locally — the .NET 10 / test-dependency bumps changed test
scheduling enough for the latent race to surface on CI. There is no product or test-logic
regression: the build, packaging, and the other 91 tests all passed with 0 warnings.

Fix

Put the two SelfLog-sharing test classes in a single xUnit collection with
DisableParallelization = true so they execute sequentially against each other, eliminating
the overlapping window. Other collections still run in parallel, so the suite stays fast.

• Add Support/SelfLogCollection.cs — [CollectionDefinition("SelfLog", DisableParallelization = true)]
• [Collection("SelfLog")] on SerilogLoggerTests
• [Collection("SelfLog")] on EnricherTests

Validation

I could not reproduce the failing tests on my dev machine so this needs to be validated in the pipeline runs.

Notes

• This makes the tests run a bit slower as some tests are serialized instead of parallel
• Current version of xUnit in the solution is depricated, migration to v3 should be done when possible but is not a blocker for future work in this repo. Info: Migration guide

[KodrAus]

Thanks @thekim1!