Disable LTO to fix self test failures
    
    LTO appears to break hotkey tests in GuiManager test suite (we had a
    patch that disabled the failing tests). I don't know if LTO is breaking
    the self test, or if the self tests are catching an actual issue in
    LTO'd 0ad.
    
    This commit disables LTO to be on the safe side, and re-enables self
    tests.