python-utils-r1.eclass: epytest, error out on missing async plugin

Explicitly error out if epytest is run without an appropriate async
plugin, and the test suite contains async tests.  Currently, these tests
are skipped with a warning but that is usually a mistake, and one can
easily miss it when pytest-asyncio or a similar plugin is installed
on the test system.  However, a missing dependency can result
in the tests being skipped afterwards on the tinderbox.

Signed-off-by: Michał Górny <mgorny@gentoo.org>

eclass/python-utils-r1.eclass

@@ -1349,6 +1349,9 @@ epytest() {
 		# override filterwarnings=error, we do not really want -Werror
 		# for end users, as it tends to fail on new warnings from deps
 		-Wdefault
+		# however, do error out if the package failed to load
+		# an appropriate async plugin
+		-Werror::pytest.PytestUnhandledCoroutineWarning
 		# override color output
 		"--color=${color}"
 		# count is more precise when we're dealing with a large number