test(flaky): avoid relying on race condition for tested behavior #6452

## Problem
#6451

This rename test relies on a specific result of a race condition for the
expected result.
- The test is checking for a telemetry result that is only emitted when
`fs.exists` takes more than 1 attempt to resolve to true.
- Therefore, it wants the first `fs.exists` check to fail, then a
subsequent one to succeed.
- It does this by not awaiting the result, and then writing the file to
be renamed.

Usually this is fine, but it is possible that the write (`toFile`)
happens before the read (`fs.exists`) since neither is awaited. This
behavior leads to a flaky test as described in the issue.

## Solution
- use a stub to force the first call to `fs.exists` to fail. 
- allow all other calls to "go through" to the original function.

packages/core/src/test/shared/fs/fs.test.ts

@@ -411,16 +411,20 @@ describe('FileSystem', function () {
             const oldPath = testFolder.pathFrom('oldFile.txt')
             const newPath = testFolder.pathFrom('newFile.txt')
 
-            const result = fs.rename(oldPath, newPath)
-            // this file is created after the first "exists" check fails, the following check should pass
-            void testutil.toFile('hello world', oldPath)
-            await result
+            const existsStub = Sinon.stub(FileSystem.prototype, 'exists')
+            existsStub.onFirstCall().resolves(false)
+            existsStub.callThrough()
+
+            await testutil.toFile('hello world', oldPath)
+            await fs.rename(oldPath, newPath)
 
             testutil.assertTelemetry('ide_fileSystem', {
                 action: 'rename',
                 result: 'Succeeded',
                 reason: 'RenameRaceCondition',
             })
+
+            existsStub.restore()
         })
     })