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.

htlcswitch/link_isolated_test.go

@@ -95,7 +95,7 @@ func (l *linkTestContext) receiveHtlcAliceToBob() {
 func (l *linkTestContext) sendCommitSigBobToAlice(expHtlcs int) {
 	l.t.Helper()
 
-	testQuit, testQuitFunc := context.WithCancel(context.Background())
+	testQuit, testQuitFunc := context.WithCancel(l.t.Context())
 	defer testQuitFunc()
 	sigs, err := l.bobChannel.SignNextCommitment(testQuit)
 	if err != nil {