move more testing logic into scripts

[jameslamb]

Contributes to #115.

Some testing details like how to run mypy and pytest are currently repeated across CI configuration and the documentation. Once conda builds and corresponding tests are added, there will be even more CI jobs that need to run those same commands.

To reduce duplication and unnecessary differences between the way tests are run locally vs. in CI, this proposes moving that logic into scripts in the ci/ directory.

Notes for Reviewers

These script names follow conventions in use across RAPIDS, which are roughly like this:

• ci/run_pytests*.sh = run tests, without any reliance on CI-specific details (intended to be usable in local development)
• ci/test_*.sh = invoke ci/run_pytests*.sh, but maybe with other CI-specific details such as creating a dedicated conda environment
  • *(to be introduced in a later PR, like add conda builds #129)