multi: context.Background() -> t.Context()

Use the new feature of Go 1.24, fix linter warnings.

This change was produced by:
 - running golangci-lint run --fix
 - sed 's/context.Background/t.Context/' -i `git grep -l context.Background | grep test.go`
 - manually fixing broken tests
 - itest, lntest: use ht.Context() where ht or hn is available
 - in HarnessNode.Stop() we keep using context.Background(), because it is
   called from a cleanup handler in which t.Context() is canceled already.

contractcourt/chain_watcher_test.go

@@ -173,7 +173,7 @@ func TestChainWatcherRemoteUnilateralClosePendingCommit(t *testing.T) {
 
 	// With the HTLC added, we'll now manually initiate a state transition
 	// from Alice to Bob.
-	testQuit, testQuitFunc := context.WithCancel(context.Background())
+	testQuit, testQuitFunc := context.WithCancel(t.Context())
 	t.Cleanup(testQuitFunc)
 	_, err = aliceChannel.SignNextCommitment(testQuit)
 	require.NoError(t, err)