Fix System.Runtime tests for single file runner (dotnet#50996)

Allows us to test the documented single file behaviors.

Not adding a new `PlatformDetection` thing for this since we don't really have a way to detect single file that wouldn't end us up testing a tautology.

eng/testing/tests.singlefile.targets

@@ -2,6 +2,8 @@
   <PropertyGroup>
     <OutputType>Exe</OutputType>
 
+    <DefineConstants>$(DefineConstants);SINGLE_FILE_TEST_RUNNER</DefineConstants>
+
     <BundleDir>$([MSBuild]::NormalizeDirectory('$(OutDir)', 'publish'))</BundleDir>
     <RunScriptOutputPath>$([MSBuild]::NormalizePath('$(BundleDir)', '$(RunScriptOutputName)'))</RunScriptOutputPath>
     <RuntimeIdentifier>$(PackageRID)</RuntimeIdentifier>

src/libraries/System.Runtime/tests/System/Reflection/ModuleTests.cs

@@ -81,6 +81,9 @@ public void CustomAttributes()
         [Fact]
         public void FullyQualifiedName()
         {
+#if SINGLE_FILE_TEST_RUNNER
+            Assert.Equal("<Unknown>", Module.FullyQualifiedName);
+#else
             var loc = AssemblyPathHelper.GetAssemblyLocation(Assembly.GetExecutingAssembly());
 
             // Browser will include the path (/), so strip it
@@ -90,12 +93,17 @@ public void FullyQualifiedName()
             }
 
             Assert.Equal(loc, Module.FullyQualifiedName);
+#endif
         }
 
         [Fact]
         public void Name()
         {
+#if SINGLE_FILE_TEST_RUNNER
+            Assert.Equal("<Unknown>", Module.Name, ignoreCase: true);
+#else
             Assert.Equal("system.runtime.tests.dll", Module.Name, ignoreCase: true);
+#endif
         }
 
         [Fact]