[wasm] Fix build errors due to trimming in System.Reflection.Metadata…

…LoadContext

The test project explicitly copies, and needs `mscorlib.dll`, which gets
trimmed out with `EnableAggressiveTrimming=true`. Preserve that.

```
error MSB3030: Could not copy the file "/__w/1/s/artifacts/bin/System.Reflection.MetadataLoadContext.Tests/net6.0-Release/browser-wasm/publish/mscorlib.dll" because it was not found.
    [/__w/1/s/src/libraries/System.Reflection.MetadataLoadContext/tests/System.Reflection.MetadataLoadContext.Tests.csproj]
```

src/libraries/System.Reflection.MetadataLoadContext/tests/ILLink.Descriptors.xml

@@ -0,0 +1,3 @@
+<linker>
+  <assembly fullname="mscorlib" />
+</linker>

src/libraries/System.Reflection.MetadataLoadContext/tests/System.Reflection.MetadataLoadContext.Tests.csproj

@@ -78,5 +78,7 @@
     <WasmFilesToIncludeFromPublishDir Include="System.Private.CoreLib.dll" />
     <WasmFilesToIncludeFromPublishDir Include="System.Reflection.MetadataLoadContext.Tests.dll" />
     <WasmFilesToIncludeFromPublishDir Include="mscorlib.dll" />
+
+    <TrimmerRootDescriptor Include="$(MSBuildThisFileDirectory)ILLink.Descriptors.xml" />
   </ItemGroup>
 </Project>