Disable source gen unit tests on mono.

[eiriktsarpalis]

Fix #92467.

[lewing]

You realize this isn't failing just on mono right? https://helixre107v0xdcypoyl9e7f.blob.core.windows.net/dotnet-runtime-refs-pull-97641-merge-94c64fb616f5450fa3/System.Text.Json.SourceGeneration.Roslyn4.4.Unit.Tests/1/console.ec1a3b8f.log?helixlogtype=result - Or at least that issue is picking up more failures (I see the tests are disabled on coreclr too)

[eiriktsarpalis]

Hmm, we had disabled the tests against checked builds of CoreCLR in reaction to #71962, but I wonder if we might want to disable it for x86 hardware as well.

[akoeplinger]

Yeah it looks like x86 is the common denominator here, not the runtime.

[eiriktsarpalis]

Do we have a good way of skipping x86 on test classes wholesale? Seems like only ConditionalFact and ConditionalTheory allow this on a test-by-test basis.

[eiriktsarpalis]

Regardless, I'll merge this PR for now since mono test runs are impacted regardless of architecture.

[akoeplinger]

@eiriktsarpalis you can use ConditionalClass for that.