Enable demo test

[cpunion]

• [x] Skip test c files likes go/src/crypto/internal/boring/div_test.c to avoid duplicated main
• [x] Force run demo with python 3.12
• [x] Enable demo result checking
  • [x] Fix _demo/logdemo, blocked by https://github.com/goplus/llgo/issues/877
  • [x] Fix _demo/{oslookpath,sysexec}

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 95.54%. Comparing base (93245ac) to head (1ba3474). Report is 5 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #876 +/- ## ======================================= Coverage 95.54% 95.54% ======================================= Files 22 22 Lines 6246 6246 ======================================= Hits 5968 5968 Misses 246 246 Partials 32 32 ```

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