Fix test script: Consume memory the whole time, not just the end

[asmacdo]

duct ./test_script.py -- --duration 3 --memory-size 1000                        
duct is executing ./test_script.py --duration 3 --memory-size 1000...
Log files will be written to .duct/logs/2024.06.07T10.26.03-527426_
this is of test of STDERR
this is of test of STDOUT
Test completed. Consumed 1000 MB for 3 seconds with CPU load factor 10000.

Exit Code: 0
Command: ./test_script.py --duration 3 --memory-size 1000
Log files location: .duct/logs/2024.06.07T10.26.03-527426_
Wall Clock Time: 3.3988728523254395
Memory Peak Usage: 3.1%
CPU Peak Usage: 119.0%