tests: Install pytest-subtests plugin

This plugin enables pytest to make use of with self.subTest blocks. By default the test ends as soon as first assertion fails but using subtests you can make multiple assertion even if one of them fails.

This is useful to diagnose failing tests as it allows examining the test from multiple assertion prespectives without having to make multiple tests with different assertions.

This plugin is available in Debian repositories under python3-pytest-subtests name.