chore: clean up tests (#37715)

1. use MockVariableValue as much as possible
2. use wg.Go as much as possible instead of Add/Done
3. simplify global lock's DefaultLocker logic to make it easier to test
4. introduce a general approach for getting external service config in
CI
5. remove unclear & unnecessary "t.Skip"
6. use modern generic syntax for remaining "DecodeJSON" calls
7. clarify test result for "list gitignore templates" and "list
licenses"

tests/integration/integration_test.go

@@ -7,6 +7,7 @@ import (
 	"bytes"
 	"context"
 	"encoding/base64"
+	"flag"
 	"fmt"
 	"hash"
 	"hash/fnv"
@@ -116,6 +117,11 @@ func testMain(m *testing.M) int {
 }
 
 func TestMain(m *testing.M) {
+	// -test.list must skip InitIntegrationTest, which requires a database.
+	flag.Parse()
+	if flag.Lookup("test.list").Value.String() != "" {
+		os.Exit(m.Run())
+	}
 	os.Exit(testMain(m))
 }