Tests: use preloader instead of URLClassLoader in MockLibraryUtil

This fixes runtime dependency of the compiler and thus allows to run tests
which use MockLibraryUtil in case of an ABI-incompatible change; also it's
slightly more efficient (noticeable when running a single test)

compiler/tests/compiler-tests.iml

@@ -19,6 +19,7 @@
     <orderEntry type="module" module-name="builtins-serializer" />
     <orderEntry type="module" module-name="js.frontend" />
     <orderEntry type="module" module-name="js.tests" />
+    <orderEntry type="module" module-name="preloader" />
     <orderEntry type="module" module-name="util" />
   </component>
 </module>