fix: Refactor MainTest to avoid stubbing System.exit()

[bdferris-v2]

Refactor MainTest to avoid stubbing System.exit() and remove dependency on SystemStubs, which doesn't work on JDK18+.

Closes #1409.

• Run the unit tests with gradle test to make sure you didn't break anything
• Format the title like "feat: [new feature short description]". Title must follow the Conventional Commit Specification(https://www.conventionalcommits.org/en/v1.0.0/).
• Linked all relevant issues
• Include screenshot(s) showing how this pull request works and fixes the issue(s)

[github-actions]

✅ Rule acceptance tests passed.
New Errors: 0 out of 1426 datasets (~0%) are invalid due to code change, which is less than the provided threshold of 1%.
Dropped Errors: 0 out of 1426 datasets (~0%) are invalid due to code change, which is less than the provided threshold of 1%.
New Warnings: 0 out of 1426 datasets (~0%) are invalid due to code change, which is less than the provided threshold of 1%.
Dropped Warnings: 0 out of 1426 datasets (~0%) are invalid due to code change, which is less than the provided threshold of 1%.
0 out of 1426 sources (~0 %) are corrupted.
Commit: ac99f7d
Download the full acceptance test report here (report will disappear after 90 days).
✅ Rule acceptance tests passed.

[davidgamez]

I tested it locally and worked fine. I was able to compile with JDKs 11 and 19.

[github-actions]

✅ Rule acceptance tests passed.
New Errors: 0 out of 1426 datasets (~0%) are invalid due to code change, which is less than the provided threshold of 1%.
Dropped Errors: 0 out of 1426 datasets (~0%) are invalid due to code change, which is less than the provided threshold of 1%.
New Warnings: 0 out of 1426 datasets (~0%) are invalid due to code change, which is less than the provided threshold of 1%.
Dropped Warnings: 0 out of 1426 datasets (~0%) are invalid due to code change, which is less than the provided threshold of 1%.
0 out of 1426 sources (~0 %) are corrupted.
Commit: 00ca328
Download the full acceptance test report here (report will disappear after 90 days).
✅ Rule acceptance tests passed.

[github-actions]

✅ Rule acceptance tests passed.
New Errors: 0 out of 1426 datasets (~0%) are invalid due to code change, which is less than the provided threshold of 1%.
Dropped Errors: 0 out of 1426 datasets (~0%) are invalid due to code change, which is less than the provided threshold of 1%.
New Warnings: 0 out of 1426 datasets (~0%) are invalid due to code change, which is less than the provided threshold of 1%.
Dropped Warnings: 0 out of 1426 datasets (~0%) are invalid due to code change, which is less than the provided threshold of 1%.
0 out of 1426 sources (~0 %) are corrupted.
Commit: db11534
Download the full acceptance test report here (report will disappear after 90 days).
✅ Rule acceptance tests passed.