test-ukify: skip signing in tests when slow tests are disabled

test-ukify: skip signing in tests when slow tests are disabled

I have a large initrd (built with mkosi-initrd) and the test-ukify takes 30 s.
Let's use the usual approach of skipping the slowests tests.

(pytest has marks, and it would be nicer to mark tests with pytest.mark.slow,
and then use "-m 'not slow'" in the meson test invocation. But markers must be
pre-registered, otherwise pytest emits a warning. There are a few ways to
register markers, but they all require "project configuration", but because of
how we invoke pytest, this is hard to do. So let's just use an environment
variable.)

(cherry picked from commit a181901ab2248f7359e48e0c6394a2f9dd0b6d39)
(cherry picked from commit f5780a2f67e989e9362bf0d19cc61be3962bae54)