Fix test cases for stage2.

[mathetake]

Currently CI is failing on master branch because I think I should have changed this in #9178 where I added one line in start.zig

cc @andrewrk

Unexpected error:
================================================================================
start.zig:86:9: error: struct 'test_case.test_case' has no member named 'main'
================================================================================
Expected error not found:
================================================================================
:85:9: error: struct 'test_case.test_case' has no member named 'main'
================================================================================

[andrewrk]

Thanks for the follow-up. Apologies for that brittle test. @mikdusan already implemented a solution to address this in the test harness elsewhere, but I didn't get around to adapting the solution to stage2 tests.