Add unit tests for DOTNET_ROOT forwarding logic and remote process architecture detection

[nohwnd]

This PR adds unit tests for several under-tested code paths in dotnet test host startup.

What's covered

DotnetTestHostManagerTests.cs — 7 new tests targeting early DOTNET_ROOT path-setting and legacy testhost handling:

• Verifies DOTNET_ROOT_<ARCH> is set early (before testhost.exe found) so xunit v3 child-processes pick it up
• Verifies the early-set path is skipped when the feature flag is enabled
• Verifies existing DOTNET_ROOT_<ARCH> env vars are not overwritten
• Covers the legacy (old-testhost) x64 and x86 DOTNET_ROOT paths
• Covers architecture-mismatch guard on old testhost path

ProcessHelperTests.cs (new file) — 3 tests for remote-process architecture detection:

• GetCurrentProcessArchitecture returns a valid Architecture value
• Repeated calls return the same cached result
• GetProcessArchitecture throws NotImplementedException on non-Windows (expected guard)

Why these tests

These paths had no test coverage despite being critical to test host startup on Linux/macOS and to multi-architecture scenarios. The 7-test DotnetTestHostManager block and 3-test ProcessHelper block together close a meaningful reliability gap.

[Copilot]

Pull request overview

Adds unit tests to improve coverage of dotnet testhost startup behavior (DOTNET_ROOT forwarding) and process architecture detection logic used by hang-dumper/procdump scenarios.

Changes:

• Added ProcessHelper unit tests around current/remote process architecture detection and platform guards.
• Added DotnetTestHostManager unit tests for early DOTNET_ROOT_<ARCH> forwarding and legacy DOTNET_ROOT / DOTNET_ROOT(x86) handling paths.
• Expanded InternalsVisibleTo to allow new tests to access internal FeatureFlag APIs.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
test/Microsoft.TestPlatform.TestHostProvider.UnitTests/System/ProcessHelperTests.cs	New tests for GetCurrentProcessArchitecture / GetProcessArchitecture behavior across OSes.
test/Microsoft.TestPlatform.TestHostProvider.UnitTests/Hosting/DotnetTestHostManagerTests.cs	New test coverage for early DOTNET_ROOT forwarding, feature-flag behavior, and old testhost legacy env vars.
src/Microsoft.TestPlatform.CoreUtilities/Friends.cs	Grants internals access to additional unit test assemblies needed by the new tests.

[Copilot]

GetCurrentProcessArchitecture can return additional PlatformArchitecture values (e.g., S390x/Ppc64le/RiscV64/LoongArch64). This assertion currently only allows X86/X64/ARM/ARM64, so it will fail on those architectures; consider checking Enum.IsDefined(typeof(PlatformArchitecture), arch) (or including all enum values) instead of hard-coding a subset.

Suggested change

	// Must be one of the known values — never throw.
	Assert.IsTrue(
	arch is PlatformArchitecture.X64
	or PlatformArchitecture.X86
	or PlatformArchitecture.ARM64
	or PlatformArchitecture.ARM,
	// Must be a defined PlatformArchitecture value — never throw.
	Assert.IsTrue(
	Enum.IsDefined(typeof(PlatformArchitecture), arch),

[Copilot]

This test is named/structured as an x64 scenario but it relies on DotnetTestHostManager's current TargetPlatform default (from prior Initialize call). On ARM64 (or other) agents that default won’t be X64, and the legacy DOTNET_ROOT forwarding branch won’t run (architectureFromEnv != _architecture). To make the test robust, explicitly initialize the host manager with TargetPlatform=x64 within the test (similar to the x86 test below).

Suggested change

	var path = @"C:\dotnet";
	var path = @"C:\dotnet";
	_dotnetHostManager.Initialize(
	_messageLogger,
	"<RunSettings><RunConfiguration><TargetPlatform>x64</TargetPlatform></RunConfiguration></RunSettings>");