Enable analyzers on test assemblies

[tillig]

I noticed that, while we have a set of test analysis rules, we didn't actually have static analysis running on the test assemblies.

This pull request fixes that:

• Enable static analysis on test assemblies.
• Update the Test.ruleset to turn off rules that don't make much sense in tests.
• Fix remaining analyzer issues.

The vast majority of the issues were around things being created in tests and not disposed - var x instead of using var x. It looks a little messy but it was easy enough to fix.

The second largest number was around nested public types in test fixtures. I think this is something that was hit while cleaning up tests in Autofac.Extensions.DependencyInjection or similar - I remember a bit of churn on one of those. This fixes that. I tried to just mark things private as much as possible, though there were a couple I simply suppressed the issue on rather than try to move things around too much.

There are a few suppressions, mostly around:

• Naming of test stubs that help indicate what they're for - this sometimes conflicts with "good framework naming."
• Odd constructs like set-with-no-get properties that are specifically for testing edge cases.
• Some test fixtures that have nested test fixtures sort of BDD style.
• Some stubs or methods where disposable things are getting created but it's not really clear that we can dispose the thing being created and returned.

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 50.00000% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 78.57%. Comparing base (4547d16) to head (4e0ad9f).

Files	Patch %	Lines
...tration/ExternalRegistryServiceMiddlewareSource.cs	0.00%	0 Missing and 1 partial :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## develop #1418 +/- ## =========================================== + Coverage 78.52% 78.57% +0.05% =========================================== Files 200 200 Lines 5713 5713 Branches 1168 1168 =========================================== + Hits 4486 4489 +3 + Misses 714 712 -2 + Partials 513 512 -1 ```

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