Fix some test flakiness. (#5345)

* Fix test flakiness in node-core-library.

* Fix some test flakiness in rush-lib.

Author: iclanton

common/changes/@microsoft/rush/fix-tests_2025-08-31-02-01.json

@@ -0,0 +1,10 @@
+{
+  "changes": [
+    {
+      "packageName": "@microsoft/rush",
+      "comment": "",
+      "type": "none"
+    }
+  ],
+  "packageName": "@microsoft/rush"
+}

common/changes/@rushstack/node-core-library/fix-tests_2025-08-31-01-31.json

@@ -0,0 +1,11 @@
+{
+  "changes": [
+    {
+      "comment": "",
+      "type": "none",
+      "packageName": "@rushstack/node-core-library"
+    }
+  ],
+  "packageName": "@rushstack/node-core-library",
+  "email": "iclanton@users.noreply.github.com"
+}

libraries/node-core-library/src/test/FileSystem.test.ts

@@ -43,10 +43,9 @@ describe(FileSystem.name, () => {
     });
 
     test('Should return true for an error on a file descriptor call', () => {
-      const buffer: Buffer = Buffer.allocUnsafeSlow(1024);
       expect.assertions(1);
       try {
-        fs.readSync(11, buffer, 0, buffer.length, -1);
+        fs.readFileSync(`${__dirname}/nonexistent.txt`);
       } catch (error) {
         expect(FileSystem.isErrnoException(error)).toBe(true);
       }

libraries/rush-lib/src/cli/test/RushCommandLineParser.test.ts

@@ -47,6 +47,10 @@ describe('RushCommandLineParser', () => {
     afterEach(() => {
       jest.clearAllMocks();
       EnvironmentConfiguration.reset();
+      jest
+        .spyOn(EnvironmentConfiguration, 'buildCacheOverrideJsonFilePath', 'get')
+        .mockReturnValue(undefined);
+      jest.spyOn(EnvironmentConfiguration, 'buildCacheOverrideJson', 'get').mockReturnValue(undefined);
     });
 
     describe('in basic repo', () => {