Fix test.sh to not modify .pylintrc.

[carlosgmartin]

Addresses #1024 (comment) and #1067 by editing test.sh to not modify or delete .pylintrc.

Also applies other improvements to test.sh:

• Do not enable xtrace (set -x), to reduce the verbosity of the output
• Replace pip commands with python3 -m pip
  • bpo-22295: use python -m pip rather than plain pip in examples python/cpython#24003
  • Clarify available commands for package installation python/cpython#66491
• Replace `command` with $(command)
  • Why is $(...) preferred over `...` (backticks)?

[fabianp]

thanks!