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.

lntest/fee_service.go

@@ -1,7 +1,6 @@
 package lntest
 
 import (
-	"context"
 	"encoding/json"
 	"fmt"
 	"io"
@@ -130,7 +129,7 @@ func (f *FeeService) handleRequest(w http.ResponseWriter, _ *http.Request) {
 
 // Stop stops the web server.
 func (f *FeeService) Stop() error {
-	err := f.srv.Shutdown(context.Background())
+	err := f.srv.Shutdown(f.Context())
 	require.NoError(f, err, "cannot stop fee api")
 
 	f.wg.Wait()