Further investigation on 32 bit CI build thread behavior in tests

The multithreaded tests for query plan were failing using 1000 threads in 32 bit mode, but passing in 64 bit mode. Attempts to simplify the tests to debug, caused the failures to occur in other places in these tests. Reducing the threads from 1000 to 500 made the tests pass in 32 bit mode. For now, we'll use the higher number of threads in 64 bit tests, but cap them at 500 for 32 bit.