pillar/tests: Fix deadlock when Debug Server startup fails.

In `startIntegratedPSICollectorAPI`, if the server does not start within
10 seconds, the function returns an error without unlocking
`psiServerMutex`. This oversight leads to a deadlock in subsequent tests
that attempt to acquire the same mutex.

This commit adds `psiServerMutex.Unlock()` before returning the error to
ensure that the mutex is properly released even when the server fails to
start.

Signed-off-by: Nikolay Martyanov <nikolay@zededa.com>

pkg/pillar/agentlog/agentlog_test.go

@@ -422,6 +422,7 @@ func startIntegratedPSICollectorAPI() (cancel context.CancelFunc, err error) {
 		time.Sleep(100 * time.Millisecond)
 	}
 	if !started {
+		psiServerMutex.Unlock()
 		return nil, fmt.Errorf("could not start the server in 10 seconds")
 	}
 	ctx, cancel := context.WithCancel(context.Background())