test(dev-infra): extract commit message build function into testing util function (#41476)

Creates a testing utility function to build commit message strings.

PR Close #41476

dev-infra/commit-message/parse.spec.ts

@@ -7,9 +7,9 @@
  */
 
 import {parseCommitMessage} from './parse';
+import {commitMessageBuilder, CommitMessageParts} from './test-util';
 
-
-const commitValues = {
+const commitValues: CommitMessageParts = {
   prefix: '',
   type: 'fix',
   npmScope: '',
@@ -19,12 +19,7 @@ const commitValues = {
   footer: 'Closes #1',
 };
 
-function buildCommitMessage(params: Partial<typeof commitValues> = {}) {
-  const {prefix, npmScope, type, scope, summary, body, footer} = {...commitValues, ...params};
-  const scopeSlug = npmScope ? `${npmScope}/${scope}` : scope;
-  return `${prefix}${type}${scopeSlug ? '(' + scopeSlug + ')' : ''}: ${summary}\n\n${body}\n\n${
-      footer}`;
-}
+const buildCommitMessage = commitMessageBuilder(commitValues);
 
 
 describe('commit message parsing:', () => {