fix(tests): "rejected promise … FileSystemError … big-image.jpg"

Problem:
Unhandled promise rejection in test logs:

    rejected promise not handled within 1 second: AssertionError (FileSystemError): No file found for key "big-image.jpg"
    stack trace: AssertionError (FileSystemError): No file found for key "big-image.jpg"
        at Function.e (/…/.vscode-test/vscode-linux-x64-1.85.1/resources/app/out/vs/workbench/api/node/extensionHostProcess.js:143:27896)

The tests that open the `bigImage` file don't actually care about its
content (and thus didn't fail); when the provider (asynchronously,
later) tries to load the file, it gets an error.

Solution:
Add the file to the fake storage service, via `s3.addFile()`.

Author: justinmk3

src/test/s3/util/fileViewerManager.test.ts

@@ -204,6 +204,9 @@ describe('FileViewerManager', function () {
         })
 
         disposables = registerFileSystemProviders()
+
+        const bigImageFile = makeFile('big-image.jpg', Buffer.from('fake image', 'utf-8'))
+        s3.addFile(bigImageFile)
     })
 
     afterEach(async function () {