gopkg.in/check.v1 hasn't had any commit since Nov 2020.
That's not a immediate issue for a test-only dependency, but
because it hides access to the standard library *testing.T,
eventually it will become limiting.
Also, using the same framework for unit and integration tests
seems practical.
This is mostly a batch copy&paste job, with a fairly high risk
of unexpected breakage.
Also, I didn't take much time at all to carefully choose between
assert.* and require.*; we can tune that as failures show up.
Signed-off-by: Miloslav Trmač <mitr@redhat.com>