Unit test failures when run from VS

[JanKrivanek]

Issue Description

Some of the unit tests are failing when run from VS (e.g. UnusedEnvironmentVariablesDoNotAppearInBinaryLog).

Steps to Reproduce

Run UnusedEnvironmentVariablesDoNotAppearInBinaryLog unit test in VS

Expected Behavior

Test passes

Actual Behavior

Test fails

Analysis

This is caused by the fact that those tests are supposed to execute msbuild and fail to do so due to attempts to interpret the test runner as the dotnet host process:

msbuild/src/UnitTests.Shared/RunnerUtilities.cs

Lines 56 to 66 in cc3db35

	/// <summary>
	/// Resolve the platform specific path to the runtime executable that msbuild.exe needs to be run in (unix-mono, {unix, windows}-corerun).
	/// </summary>
	private static string ResolveRuntimeExecutableName()
	{
	// Run the child process with the same host as the currently-running process.
	using (Process currentProcess = Process.GetCurrentProcess())
	{
	return currentProcess.MainModule.FileName;
	}
	}

Proposal

This is how sdk achieves similar thing: https://github.com/dotnet/sdk/blob/main/src/Resolvers/Microsoft.DotNet.NativeWrapper/EnvironmentProvider.cs#L65-L94

Although it's sdk specific, it should be sufficient, since the code is supposed to run only for core:

msbuild/src/Directory.BeforeCommon.targets

Line 54 in cc3db35

<DefineConstants>$(DefineConstants);FEATURE_RUN_EXE_IN_TESTS</DefineConstants>