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.

routing/localchans/manager_test.go

@@ -317,7 +317,7 @@ func TestManager(t *testing.T) {
 			expectedNumUpdates = test.expectedNumUpdates
 
 			failedUpdates, err := manager.UpdatePolicy(
-				context.Background(),
+				t.Context(),
 				test.newPolicy,
 				test.createMissingEdge,
 				test.specifiedChanPoints...)