ObjPtr<>-ify tests using ClassLinker::FindClass().

ClassLinker::FindClass() returns a non-ObjPtr<> reference
but it has a lot of uses, so we shall change the uses in
a few steps. This change deals with several tests.

Test: Rely on TreeHugger.
Bug: 31113334
Change-Id: Ib75e20e7ebaff01fb607a09f96675f8cf397ae52
diff --git a/compiler/jni/jni_compiler_test.cc b/compiler/jni/jni_compiler_test.cc
index c643af7..3cb4a65 100644
--- a/compiler/jni/jni_compiler_test.cc
+++ b/compiler/jni/jni_compiler_test.cc
@@ -245,7 +245,7 @@
     Handle<mirror::ClassLoader> loader(
         hs.NewHandle(soa.Decode<mirror::ClassLoader>(class_loader)));
     // Compile the native method before starting the runtime
-    mirror::Class* c = class_linker_->FindClass(soa.Self(), "LMyClassNatives;", loader);
+    ObjPtr<mirror::Class> c = class_linker_->FindClass(soa.Self(), "LMyClassNatives;", loader);
     const auto pointer_size = class_linker_->GetImagePointerSize();
     ArtMethod* method = c->FindClassMethod(method_name, method_sig, pointer_size);
     ASSERT_TRUE(method != nullptr) << method_name << " " << method_sig;