test: fix flaky test-runner coverage threshold test

Reset process.exitCode before running the coverage threshold check.
This lets the test verify the exit code set by that run() call without
depending on process-global state from earlier coverage runs.

Signed-off-by: Kamat, Trivikram <16024985+trivikr@users.noreply.github.com>
PR-URL: #64051
Refs: https://github.com/nodejs/reliability/issues?q=%22test-runner-run-coverage%22
Reviewed-By: Chemi Atlow <chemi@atlow.co.il>
Reviewed-By: Filip Skokan <panva.ip@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Jacob Smith <jacob@frende.me>

Author: trivikr

test/parallel/test-runner-run-coverage.mjs

@@ -154,10 +154,13 @@ describe('require(\'node:test\').run coverage settings', { concurrency: true },
       for await (const _ of stream);
     });
 
-    await it('should run with coverage and fail when below line threshold', async () => {
+    await it('should run with coverage and fail when below line threshold', async (t) => {
       const thresholdErrors = [];
       const originalExitCode = process.exitCode;
-      assert.notStrictEqual(originalExitCode, 1);
+      t.after(() => {
+        process.exitCode = originalExitCode;
+      });
+      process.exitCode = undefined;
       const stream = run({
         files,
         coverage: true,
@@ -178,7 +181,6 @@ describe('require(\'node:test\').run coverage settings', { concurrency: true },
       for await (const _ of stream);
       assert.deepStrictEqual(thresholdErrors.sort(), ['branch', 'function', 'line']);
       assert.strictEqual(process.exitCode, 1);
-      process.exitCode = originalExitCode;
     });
   });
 });