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.

channeldb/addr_source_test.go

@@ -20,7 +20,7 @@ var (
 // deduplicates the results of a set of AddrSource implementations.
 func TestMultiAddrSource(t *testing.T) {
 	t.Parallel()
-	ctx := context.Background()
+	ctx := t.Context()
 
 	var pk1 = newTestPubKey(t)
 

channeldb/db_test.go

@@ -1,7 +1,6 @@
 package channeldb
 
 import (
-	"context"
 	"image/color"
 	"math"
 	"math/rand"
@@ -181,7 +180,7 @@ func TestFetchClosedChannelForID(t *testing.T) {
 // channel db and graph db.
 func TestMultiSourceAddrsForNode(t *testing.T) {
 	t.Parallel()
-	ctx := context.Background()
+	ctx := t.Context()
 
 	fullDB, err := MakeTestDB(t)
 	require.NoError(t, err, "unable to make test database")