Reactivate JsonSerializer polymorphic tests

[layomia]

Following #33391, the static test methods were not being run.

dotnet msbuild /t:test /p:xunitmethodname=*PolymorphicTests*

Before this PR

System.Text.Json.Tests Total: 50, Errors: 0, Failed: 0, Skipped: 0, Time: 0.297s

After

System.Text.Json.Tests Total: 85, Errors: 0, Failed: 0, Skipped: 0, Time: 0.327s

[stephentoub]

Wait, I'm not understanding. Are you saying xunit is no longer running static test methods in general? Or is there something specific to these cases? Thanks.

[layomia]

@stephentoub, #33391 introduced a pattern where the class containing the test methods is abstract, and test variants derive from this class. This approach allows us to test multiple overloads of the serializer with the same logic.

The side effect of this seems to be that static test methods, and methods with the [Theory] attribute are not run. Static test methods are ignored, and xunit throws a NotSupportedException for theories.

In general, tests that don't use this pattern should run as expected.

[stephentoub]

introduced a pattern where the class containing the test methods is abstract, and test variants derive from this class

Ah, ok. Phew :)