integ-runner: "Could not checkout snapshot directory" if HEAD branch does not contain snapshots

### Describe the bug

Running `integ-runner` results in a git failures:

```
Running test /Users/jvalore/Projects/mindful-composer2/test/integ.getConfiguration.ts in us-east-1
error: pathspec 'integ.getConfiguration.ts.snapshot' did not match any file(s) known to git
SUCCESS integ.getConfiguration-GetAppConfigurationLambda/DefaultTest 248.229s
AssertionResultsLambdaInvoked2f382cf205e7f04c4f85bed1cc026bf - success
AssertionResultsLambdaInvoke3c6d8322bda64b60f1d6ee49af533865 - success

Test Results:

Tests: 1 passed, 1 total
Could not checkout snapshot directory 'test/integ.getConfiguration.ts.snapshot'. Please verify the following command completes correctly:
git checkout $(git merge-base HEAD main) -- integ.getConfiguration.ts.snapshot
error: Error: Command exited with status 1
at exec2 (/Users/jvalore/Projects/mindful-composer2/node_modules/@aws-cdk/integ-runner/lib/workers/extract/index.js:12932:11)
at IntegTestRunner.checkoutSnapshot (/Users/jvalore/Projects/mindful-composer2/node_modules/@aws-cdk/integ-runner/lib/workers/extract/index.js:20124:13)
at IntegTestRunner.deploy (/Users/jvalore/Projects/mindful-composer2/node_modules/@aws-cdk/integ-runner/lib/workers/extract/index.js:20420:18)
at IntegTestRunner.runIntegTestCase (/Users/jvalore/Projects/mindful-composer2/node_modules/@aws-cdk/integ-runner/lib/workers/extract/index.js:20199:37)
at Function.integTestWorker (/Users/jvalore/Projects/mindful-composer2/node_modules/@aws-cdk/integ-runner/lib/workers/extract/index.js:30637:34)
at MessagePort. (/Users/jvalore/Projects/mindful-composer2/node_modules/@aws-cdk/integ-runner/lib/workers/extract/index.js:967:31)
at [nodejs.internal.kHybridDispatch] (node:internal/event_target:807:20)
at exports.emitMessage (node:internal/per_context/messageport:23:28)
```

The message:
```
Could not checkout snapshot directory 'test/integ.getConfiguration.ts.snapshot'. Please verify the following command completes correctly:
git checkout $(git merge-base HEAD main) -- integ.getConfiguration.ts.snapshot
```
Looks like it doesn't include the `test/`path in the `git` command.

### Expected Behavior

Test should pass and update snapshot. (Ideally there shouldn't be `git` lock-in here either, but I guess that is a different discussion).

### Current Behavior

(See error message above)

### Reproduction Steps

Have an test in the default `test/` directory. In my case `test/integ.getConfiguration.ts`

Run `integ-runner --force` once. The snapshot is created in
```
test/integ.getConfiguration.ts.snapshot
```

`git add` and `git commit` the snapshot, as is recommended in the integ-runner docs.

Run `integ-runner --force` again.
Results in the error message listed above.

### Possible Solution

_No response_

### Additional Information/Context

_No response_

### CDK CLI Version

2.129.0 (build d5ab0df)

### Framework Version

_No response_

### Node.js Version

v20.9.0

### OS

Mac

### Language

TypeScript

### Language Version

4.8.4

### Other information

_No response_

Reproduce the failure with integ-runner --force using a test in the default test/ directory, then inspect the integ-runner checkoutSnapshot entry point and the reported command. Verify the checkout handles test/integ.getConfiguration.ts.snapshot correctly and rerun the integration test to confirm the snapshot update completes without the git error.

Written by the indexing model from the issue text.