tests: add QUBES_TEST_TEMPLATES env variable
tests: add QUBES_TEST_TEMPLATES env variable

Allow easily list templates to be tested, without enumerating all the
test classes. This is especially useful with nose2 runner which can't
use load tests protocol _and_ select subset of tests.

marmarek/qubes-core-admin,marmarek/qubes-core-admin,QubesOS/qubes-core-admin,marmarek/qubes-core-admin,QubesOS/qubes-core-admin,QubesOS/qubes-core-admin