[wasm] fix test failing when trimming

```
[06:37:05] fail: [FAIL] System.Diagnostics.Tests.StopwatchTests.DebuggerAttributesValid
[06:37:05] info: System.InvalidOperationException : Expected one DebuggerDisplayAttribute on System.Diagnostics.Stopwatch.
[06:37:05] info:    at System.Diagnostics.DebuggerAttributes.ValidateDebuggerDisplayReferences(Object obj)
[06:37:05] info:    at System.Diagnostics.Tests.StopwatchTests.DebuggerAttributesValid()
[06:37:05] info:    at System.Reflection.MethodInvoker.InterpretedInvoke(Object , Span`1 , BindingFlags )
```

Fixed by preserving the `DebuggerDisplayAttribute`.

src/libraries/System.Runtime.Extensions/tests/ILLink.Descriptors.xml

@@ -4,4 +4,7 @@
   <assembly fullname="VoidMainWithExitCodeApp" />
   <assembly fullname="TestApp" />
   <assembly fullname="TargetFrameworkNameTestApp" />
+  <assembly fullname="System.Private.CoreLib">
+    <type fullname="System.Diagnostics.DebuggerDisplayAttribute" />
+  </assembly>
 </linker>