[dagit] Fix AssetView flakiness (#10293)

### Summary & Motivation

I'm pretty sure the `Math.random()` I added to AssetView is causing flakiness. If I just make sure that only one materialization and no observations are returned for the mocked Asset, that should repair the duplicate key warning and also cause the tests to handle the materialization timestamp accurately.

### How I Tested These Changes

yarn jest AssetView

js_modules/dagit/packages/core/src/assets/AssetView.test.tsx

@@ -11,18 +11,16 @@ import {AssetView} from './AssetView';
 jest.mock('../graph/asyncGraphLayout', () => ({}));
 
 describe('AssetView', () => {
-  const defaultMocks = {
+  const mocks = {
     AssetNode: () => ({
       partitionKeys: () => [...new Array(0)],
     }),
     Asset: () => ({
-      assetMaterializations: () => [...new Array(2)],
+      assetMaterializations: () => [...new Array(1)],
+      assetObservations: () => [...new Array(0)],
     }),
     MaterializationEvent: () => ({
-      timestamp: () => `${Math.random() * 100}`,
-    }),
-    ObservationEvent: () => ({
-      timestamp: () => `${Math.random() * 100}`,
+      timestamp: () => '100',
     }),
     MetadataEntry: () => ({
       __typename: 'TextMetadataEntry',
@@ -32,7 +30,7 @@ describe('AssetView', () => {
 
   const Test = ({path}: {path: string}) => {
     return (
-      <TestProvider apolloProps={{mocks: defaultMocks}}>
+      <TestProvider apolloProps={{mocks}}>
         <MemoryRouter initialEntries={[path]}>
           <AssetView assetKey={{path: ['foo']}} />
         </MemoryRouter>