integration/libnetwork: make delInterface() a t.Helper()

Otherwise errors within this function will all show to be at the line
number of the utility, instead of where it failed in the test:

    === RUN   TestDaemonDefaultNetworkPools
        service_test.go:23: assertion failed:
            Command:  ip link delete docker0
            ExitCode: 127
            Error:    exec: "ip": executable file not found in $PATH
            Stdout:
            Stderr:

            Failures:
            ExitCode was 127 expected 0
            Expected no error

    === RUN   TestDaemonRestartWithExistingNetwork
        service_test.go:23: assertion failed:
            Command:  ip link delete docker0
            ExitCode: 127
            Error:    exec: "ip": executable file not found in $PATH
            Stdout:
            Stderr:

            Failures:
            ExitCode was 127 expected 0
            Expected no error

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

integration/network/service_test.go

@@ -20,6 +20,7 @@ import (
 
 // delInterface removes given network interface
 func delInterface(t *testing.T, ifName string) {
+	t.Helper()
 	icmd.RunCommand("ip", "link", "delete", ifName).Assert(t, icmd.Success)
 	icmd.RunCommand("iptables", "-t", "nat", "--flush").Assert(t, icmd.Success)
 	icmd.RunCommand("iptables", "--flush").Assert(t, icmd.Success)