fix(tests): mock fetch for LabelValueInput tests

Gettings lots of nodejs warnings about unhandled fetch errors due to lack of mocks for suggestion fetch requests, add mocks to supress those

ui/src/Components/SilenceModal/SilenceMatch/LabelValueInput.test.js

@@ -14,6 +14,10 @@ import { LabelValueInput } from "./LabelValueInput";
 let silenceFormStore;
 let matcher;
 
+beforeAll(() => {
+  fetch.mockResponse(JSON.stringify([]));
+});
+
 beforeEach(() => {
   silenceFormStore = new SilenceFormStore();
   matcher = NewEmptyMatcher();
@@ -28,6 +32,10 @@ beforeEach(() => {
   ];
 });
 
+afterEach(() => {
+  jest.restoreAllMocks();
+});
+
 const ShallowLabelValueInput = isValid => {
   return shallow(
     <LabelValueInput