test_runner: use exports in coverage mock fixture

Replace the deprecated `namedExports` option in the
coverage-with-mock-cjs output fixture with `exports`.

The old fixture started emitting a deprecation warning, which
changed stderr and broke the TAP snapshot for the coverage
output test. Using `exports` keeps the fixture behavior the
same while avoiding unrelated warning noise.

Author: Han5991

test/fixtures/test-runner/output/coverage-with-mock-cjs.mjs

@@ -1,7 +1,7 @@
 import { mock, test } from 'node:test';
 
 const dependency = mock.fn(() => 'mock-return-value');
-mock.module('../coverage-with-mock/dependency.cjs', { namedExports: { dependency } });
+mock.module('../coverage-with-mock/dependency.cjs', { exports: { dependency } });
 
 const { subject } = await import('../coverage-with-mock/subject.mjs');