Improve synthetic return pad for the start-at-main option

[privat]

• Use Exit2 instead of Exit, so main can return a value
• Move the return pad away so it makes the code and memory visualization cleaner, and avoid a fall through behavior
• Add tests (and activate the start-at-main option in the tests)

[github-actions]

Test Results

12 files  ±0  12 suites  ±0   34s ⏱️ -2s
 8 tests ±0   8 ✅ ±0  0 💤 ±0  0 ❌ ±0 
24 runs  ±0  24 ✅ ±0  0 💤 ±0  0 ❌ ±0 

Results for commit a8a4ba0. ± Comparison against base commit 26d11a8.