Change tests.sh for portability

[slycordinator]

1. Changed to use /bin/sh instead of /bin/bash

Everything in the script is already posix compliant.

2. Change to use find -exec rm -rf {} + instead of find | xargs rm -rf

This avoids the edge case of when one of the paths to the subdirectories have spaces or other unusual characters.

An alternative fix for that is to use "find . -name pycache -type d -print0 | xargs -0 rm -rf", but this is a less portable option