[Ekapkgs/python] Move tests to separate passthru drv

[jonringer]

Some test suites are pretty exhausitve and annoying to run. I think the default should be:

• Ensure that runtime dependencies are available post install
• Some type of import smoke test (e.g. pythonImportCheck)

By default, I would like to:

• Have a "test" output
• Move the "$testDir" to the test output
• Have some pytestCheckHook like mkDerivation which will effectively run:

  pkgs.runCommand "${drv.name}-test" { nativeBuildInputs = [ pytestCheckHook ]; } ''
    touch $out
  ''