tests: Drop pytest-split-tests in favor of pytest-split

The plugin pytest-split-tests is used to split the tests in several
groups so that the tests can be executed in parallel is upstream CI.

The issue is thet pytest-split-tests last release was more that a
year ago, upstream developers have not been responsive, and there is
a bug when the number of tests to be executed by a group is zero.
The patch to fix this issue has been open for a year.

This patch deprecates the use of pytest-split-tests, changing the
plugin used to split the tests to pytest-split, which is actively
mainatined.