Disable Client for probes test

If it were to try and export any metrics it would throw this error:

```
/home/semaphore/appsignal-nodejs/node_modules/@opentelemetry/sdk-metrics/build/src/export/PeriodicExportingMetricReader.js:76
                throw new Error(`PeriodicExportingMetricReader: metrics export failed (error ${result.error})`);
                      ^
Error: PeriodicExportingMetricReader: metrics export failed (error Error: connect ECONNREFUSED 127.0.0.1:8099)
    at doExport (/home/semaphore/appsignal-nodejs/node_modules/@opentelemetry/sdk-metrics/src/export/PeriodicExportingMetricReader.ts:133:15)
    at processTicksAndRejections (node:internal/process/task_queues:95:5)
```

src/__tests__/probes.test.ts

@@ -77,6 +77,9 @@ describe("Probes", () => {
         enableMinutelyProbes
       })
       expect(client.metrics()).toBeInstanceOf(Metrics)
+      // Stop client so it doesn't actually export any metrics, which would
+      // cause the entire test to fail as the agent's not running
+      client.stop()
 
       probes = client.metrics().probes()
     }