test: use random free grpc port when starting the manager (#5467)

* test: use random free grpc port when starting the manager

by default a manager uses port 6789 but that might be already in use
(for example by an elastic-agent already running on the system ;))

set port to 0 so the kernel can pick a free port each time

* Update manager_fake_input_test.go

* Update manager_fake_input_test.go

---------

Co-authored-by: Blake Rouse <blake.rouse@elastic.co>

Author: kruskall

internal/pkg/agent/application/coordinator/coordinator_test.go

@@ -893,7 +893,9 @@ func createCoordinator(t *testing.T, ctx context.Context, opts ...CoordinatorOpt
 	require.NoError(t, err)
 
 	monitoringMgr := newTestMonitoringMgr()
-	rm, err := runtime.NewManager(l, l, ai, apmtest.DiscardTracer, monitoringMgr, configuration.DefaultGRPCConfig(), false)
+	cfg := configuration.DefaultGRPCConfig()
+	cfg.Port = 0
+	rm, err := runtime.NewManager(l, l, ai, apmtest.DiscardTracer, monitoringMgr, cfg, false)
 	require.NoError(t, err)
 
 	caps, err := capabilities.LoadFile(paths.AgentCapabilitiesPath(), l)