Avoid running test assemblies in parallel in the same xunit process

[ericstj]

As mentioned in #11063 (comment)

We shouldn't be relying on concurrent execution of test assemblies in the xunit runner on non .NETFramework. We've hit multiple issues because of this:
#11320
#11063

We've gotten some patches into xunit to avoid these, but we should avoid the to get on the "happy path" for this component otherwise we're likely to hit more issues.

I believe the primary place where we do pass multiple assemblies is in the runtime tests. I believe those could be modified to use a single test assembly and avoid this case. AFICT they aren't even running tests in the Xunit process so it should be feasible to either generate them as a single assembly, or generate the wrapper in a different way so that it doesn't appear to xunit as multiple test assemblies.

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Issue Details

---

As mentioned in #11063 (comment)

We shouldn't be relying on concurrent execution of test assemblies in the xunit runner on non .NETFramework. We've hit multiple issues because of this:
#11320
#11063

We've gotten some patches into xunit to avoid these, but we should avoid the to get on the "happy path" for this component otherwise we're likely to hit more issues.

I believe the primary place where we do pass multiple assemblies is in the runtime tests. I believe those could be modified to use a single test assembly and avoid this case. AFICT they aren't even running tests in the Xunit process so it should be feasible to either generate them as a single assembly, or generate the wrapper in a different way so that it doesn't appear to xunit as multiple test assemblies.

Author:	ericstj
Assignees:	-
Labels:	area-Infrastructure-coreclr, untriaged
Milestone:	-