chore: fix classloader for tests (#857)

Author: Pekka Hyvönen

vaadin-spring/src/test/java/com/vaadin/flow/spring/instantiator/SpringInstantiatorTest.java

@@ -18,7 +18,6 @@
 import javax.servlet.ServletConfig;
 import javax.servlet.ServletContext;
 import javax.servlet.ServletException;
-
 import java.util.Collections;
 import java.util.List;
 import java.util.Locale;
@@ -48,8 +47,8 @@
 import com.vaadin.flow.function.DeploymentConfiguration;
 import com.vaadin.flow.i18n.I18NProvider;
 import com.vaadin.flow.server.ServiceInitEvent;
-import com.vaadin.flow.server.StaticFileServer;
 import com.vaadin.flow.server.StaticFileHandlerFactory;
+import com.vaadin.flow.server.StaticFileServer;
 import com.vaadin.flow.server.VaadinService;
 import com.vaadin.flow.server.VaadinServiceInitListener;
 import com.vaadin.flow.server.VaadinServletService;
@@ -186,6 +185,9 @@ protected DeploymentConfiguration createDeploymentConfiguration(
         ServletConfig config = Mockito.mock(ServletConfig.class);
         ServletContext servletContext = Mockito.mock(ServletContext.class);
 
+        Mockito.when(servletContext.getClassLoader())
+                .thenReturn(servlet.getClass().getClassLoader());
+
         ApplicationConfiguration appConfig = Mockito
                 .mock(ApplicationConfiguration.class);
         Mockito.when(appConfig.getPropertyNames())