chore: ignore RWLock from Istanbul coverage (#32478)

This periodically blocks PRs on CodeCov if the locking logic
nondeterminstically happens to have taken another path.

----

*By submitting this pull request, I confirm that my contribution is made
under the terms of the Apache-2.0 license*

packages/aws-cdk/lib/api/util/rwlock.ts

@@ -11,6 +11,7 @@ import * as path from 'path';
  * This class is not 100% race safe, but in practice it should be a lot
  * better than the 0 protection we have today.
  */
+/* istanbul ignore next: code paths are unpredictable */
 export class RWLock {
   private readonly pidString: string;
   private readonly writerFile: string;
@@ -158,6 +159,7 @@ export interface IWriterLock extends ILock {
   convertToReaderLock(): Promise<ILock>;
 }
 
+/* istanbul ignore next: code paths are unpredictable */
 async function readFileIfExists(filename: string): Promise<string | undefined> {
   try {
     return await fs.readFile(filename, { encoding: 'utf-8' });
@@ -168,13 +170,15 @@ async function readFileIfExists(filename: string): Promise<string | undefined> {
 }
 
 let tmpCounter = 0;
+/* istanbul ignore next: code paths are unpredictable */
 async function writeFileAtomic(filename: string, contents: string): Promise<void> {
   await fs.mkdir(path.dirname(filename), { recursive: true });
   const tmpFile = `${filename}.${process.pid}_${++tmpCounter}`;
   await fs.writeFile(tmpFile, contents, { encoding: 'utf-8' });
   await fs.rename(tmpFile, filename);
 }
 
+/* istanbul ignore next: code paths are unpredictable */
 async function deleteFile(filename: string) {
   try {
     await fs.unlink(filename);
@@ -186,6 +190,7 @@ async function deleteFile(filename: string) {
   }
 }
 
+/* istanbul ignore next: code paths are unpredictable */
 function processExists(pid: number) {
   try {
     process.kill(pid, 0);