Disable ReferenceResolution_RelativePaths on Linux, need better x-plat resolver mocks

src/Compilers/CSharp/Test/Symbol/Compilation/ReferenceManagerTests.cs

@@ -1346,7 +1346,8 @@ public void ResolvedReferencesCaching()
             var a2 = c2.SourceAssembly;
         }
 
-        [Fact]
+        // TODO: make x-plat (https://github.com/dotnet/roslyn/issues/6465)
+        [ConditionalFact(typeof(WindowsOnly))]
         public void ReferenceResolution_RelativePaths()
         {
             var t1 = Parse(@"

src/Test/Utilities/Shared/Mocks/TestMetadataReferenceResolver.cs

@@ -33,6 +33,10 @@ public override ImmutableArray<PortableExecutableReference> ResolveReference(str
                 if (_pathResolver != null)
                 {
                     reference = _pathResolver.ResolvePath(reference, baseFilePath);
+                    if (reference == null)
+                    {
+                        return ImmutableArray<PortableExecutableReference>.Empty;
+                    }
                 }
 
                 map = _files;