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.

itest/lnd_rpc_middleware_interceptor_test.go

@@ -220,7 +220,7 @@ func middlewareInterceptionTest(t *testing.T,
 
 	// Everything we test here should be executed in a matter of
 	// milliseconds, so we can use one single timeout context for all calls.
-	ctxb := context.Background()
+	ctxb := t.Context()
 	ctxc, cancel := context.WithTimeout(ctxb, defaultTimeout)
 	defer cancel()
 
@@ -372,7 +372,7 @@ func middlewareResponseManipulationTest(t *testing.T,
 
 	// Everything we test here should be executed in a matter of
 	// milliseconds, so we can use one single timeout context for all calls.
-	ctxb := context.Background()
+	ctxb := t.Context()
 	ctxc, cancel := context.WithTimeout(ctxb, defaultTimeout)
 	defer cancel()
 
@@ -489,7 +489,7 @@ func middlewareRequestManipulationTest(t *testing.T, node *node.HarnessNode,
 
 	// Everything we test here should be executed in a matter of
 	// milliseconds, so we can use one single timeout context for all calls.
-	ctxb := context.Background()
+	ctxb := t.Context()
 	ctxc, cancel := context.WithTimeout(ctxb, defaultTimeout)
 	defer cancel()
 
@@ -582,7 +582,7 @@ func middlewareMandatoryTest(ht *lntest.HarnessTest, node *node.HarnessNode) {
 	err = node.WaitUntilServerActive()
 	require.NoError(ht, err)
 
-	ctxb := context.Background()
+	ctxb := ht.Context()
 	ctxc, cancel := context.WithTimeout(ctxb, defaultTimeout)
 	defer cancel()