Revive LGTM C++ analysis

[BrettDong]

Summary

None

Purpose of change

LGTM C++ analysis has been timing out for months, resulting in a red X mark in all C++ pull requests. This does not look good.

Describe the solution

After doing a few test builds with different configurations, I found that excluding the tests from the analysis ensures it can complete in around two hours and a half. Including the tests adds 1.5 hours of build time and analysis time, and brings the total time to 4 hours and timeouts. So this pull request instructs LGTM to only analyze the main game and exclude the tests.

Describe alternatives you've considered

1. Use CMake and Ninja to spawn parallel compilation jobs as many as available logical CPUs, but in a real test it exhausts memory and thrashes the system
2. Use Clang compiler, which is expected to be slightly faster than GCC in compile speeds, but I ran into the problem that CodeQL database is not properly generated on LGTM.com build environment and no useful analysis is performed

Testing

LGTM C++ analysis succeeded, finishing within two hours: https://lgtm.com/logs/f4c70305b114ec88fac7a40b4b6277bb6192150e/lang:cpp

Additional context

LGTM C++ analysis may still fail on this pull request, because building the master branch still times out. But building the merge commit should succeed.