libnetwork: make tests less dependent on others

TestResolvConfHost and TestParallel both depended on a network named
"testhost" already existing in the libnetwork controller. Only TestHost
created that network, so the aforementioned tests would fail unless run
after TestHost. Fix those tests so they can be run in any order.

Signed-off-by: Cory Snider <csnider@mirantis.com>

libnetwork/libnetwork_linux_test.go

@@ -36,6 +36,23 @@ var (
 	testns  = netns.None()
 )
 
+var createTesthostNetworkOnce sync.Once
+
+func getTesthostNetwork(t *testing.T) libnetwork.Network {
+	t.Helper()
+	createTesthostNetworkOnce.Do(func() {
+		_, err := createTestNetwork("host", "testhost", options.Generic{}, nil, nil)
+		if err != nil {
+			t.Fatal(err)
+		}
+	})
+	n, err := controller.NetworkByName("testhost")
+	if err != nil {
+		t.Fatal(err)
+	}
+	return n
+}
+
 func createGlobalInstance(t *testing.T) {
 	var err error
 	defer close(start)
@@ -60,11 +77,7 @@ func createGlobalInstance(t *testing.T) {
 		},
 	}
 
-	net1, err := controller.NetworkByName("testhost")
-	if err != nil {
-		t.Fatal(err)
-	}
-
+	net1 := getTesthostNetwork(t)
 	net2, err := createTestNetwork("bridge", "network2", netOption, nil, nil)
 	if err != nil {
 		t.Fatal(err)
@@ -124,11 +137,7 @@ func TestHost(t *testing.T) {
 		}
 	}()
 
-	network, err := createTestNetwork("host", "testhost", options.Generic{}, nil, nil)
-	if err != nil {
-		t.Fatal(err)
-	}
-
+	network := getTesthostNetwork(t)
 	ep1, err := network.CreateEndpoint("testep1")
 	if err != nil {
 		t.Fatal(err)
@@ -708,11 +717,7 @@ func TestResolvConfHost(t *testing.T) {
 		}
 	}()
 
-	n, err := controller.NetworkByName("testhost")
-	if err != nil {
-		t.Fatal(err)
-	}
-
+	n := getTesthostNetwork(t)
 	ep1, err := n.CreateEndpoint("ep1", libnetwork.CreateOptionDisableResolution())
 	if err != nil {
 		t.Fatal(err)
@@ -991,10 +996,7 @@ func runParallelTests(t *testing.T, thrNumber int) {
 		}
 	}()
 
-	net1, err := controller.NetworkByName("testhost")
-	if err != nil {
-		t.Fatal(err)
-	}
+	net1 := getTesthostNetwork(t)
 	if net1 == nil {
 		t.Fatal("Could not find testhost")
 	}