gh-141794: Reduce size of compiler stress tests to fix Android warnings

[mhsmith]

This fixes the Android low memory warnings in test_ast and test_compile, previously discussed in #137186 (comment).

These warnings haven't led to a crash so far. However, the warning will only appear during the first test that brings the memory below a threshold. If a later test exhausts the memory completely, then the warning won't appear again, and the process will simply crash. This separation between the warning and the crash makes the crash much more difficult to debug.

I think this may have been what happened in #142228 (comment), which introduced some leaks. Notice in this run, the warning appeared during test_ast, and the crash happened in test_random.

• Issue: Compiling executors can result in excessively large amounts of machine code. #141794