eachdist.py did not support the installation of test packages,
(as defined by the extra_requires:test package group). As a
result, test packages were being added to dev-requirements.txt
By having eachdist.py develop install test packages, and moving
develop/test package definitions to the individual instrumentations,
it is easier to determine which packages require which dependencies
for testing purposes, and enables support for existing dependencies
that follow the extra_requires:test pattern.
Add a test to ensure readmes render properly
Also adds README.rst for testutil package to pass new test.
Co-authored-by: Christian Neumüller <christian+github@neumueller.me>
