itest: manage context within HarnessNode

This commit removes the context as a param needed when calling methods
of HarnessNode. This change moves the context management inside
HarnessNode, aside from saving us a few lines, it makes the context
creation/timeout less error-prone.

lntest/itest/lnd_amp_test.go

@@ -245,11 +245,9 @@ func testSendPaymentAMPInvoiceRepeat(net *lntest.NetworkHarness,
 			Amt: chanAmt,
 		},
 	)
-	ctxt, _ := context.WithTimeout(ctxb, defaultTimeout)
-	err = carol.WaitForNetworkChannelOpen(ctxt, chanPoint)
+	err = carol.WaitForNetworkChannelOpen(chanPoint)
 	require.NoError(t.t, err, "carol didn't report channel")
-	ctxt, _ = context.WithTimeout(ctxb, defaultTimeout)
-	err = dave.WaitForNetworkChannelOpen(ctxt, chanPoint)
+	err = dave.WaitForNetworkChannelOpen(chanPoint)
 	require.NoError(t.t, err, "dave didn't report channel")
 
 	// Create an AMP invoice of a trivial amount, that we'll pay repeatedly