Fix AddAmmo setting ammo to 99 when trying to add infinite ammo
This affected the portal mission, the crate only gave you 99 portal guns instead of infinite.
Lua scripts for engine testing. These scripts are the same as other Lua scripts
in Hedgewars, but you ought to call `EndLuaTest(TEST_SUCCESSFUL)` or
`EndLuaTest(TEST_FAILED)` when the test is completed.