[browser][coreCLR] use IsCoreCLRInterpreter

[pavelsavara]

Contributes to #123224

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[jkoritzinsky]

runtimevariant was added for Mono because the runtime would be built fundamentally differently depending on variant. That's not the case for how we build CoreCLR, so I don't think using it applies in the same way.

[Copilot]

Pull request overview

This PR contributes to issue #123224 by introducing a unified IsInterpreter property in PlatformDetection that combines checks for both Mono and CoreCLR interpreters, and updates test code to use this new property where appropriate.

Changes:

• Introduces IsInterpreter and IsNotInterpreter properties in PlatformDetection.cs that combine both IsMonoInterpreter and IsCoreClrInterpreter checks
• Updates test files to use the new IsInterpreter property or add IsCoreClrInterpreter checks where appropriate
• Improves consistency in interpreter detection across the test suite

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 2 comments.

File	Description
src/libraries/Common/tests/TestUtilities/System/PlatformDetection.cs	Adds IsInterpreter and IsNotInterpreter properties that combine both interpreter types
src/libraries/System.Text.Json/tests/System.Text.Json.Tests/Utf8JsonReaderTests.cs	Updates TestDepth test to use IsInterpreter instead of IsMonoInterpreter
src/libraries/System.Text.Json/tests/System.Text.Json.Tests/AssemblyInfo.cs	Updates ActiveIssue attribute to use IsInterpreter instead of IsMonoInterpreter
src/libraries/System.Runtime/tests/System.Runtime.Tests/System/Runtime/JitInfoTests.cs	Adds IsCoreClrInterpreter checks alongside existing IsMonoInterpreter checks

[jkotas]

CoreCLR interpreter is compiling methods just like a JIT. Is it expected that the number of methods compiled by interpreter JIT is not included in JitInfo.GetCompiledMethodCount?

[jkotas]

I would expect IsReadyToRunCompiled condition to sufficient here. Why is it not enough?