test: conftest: require_programs() automatically (MR 2483)

Pmbootstrap finds all its host dependencies during init, ensure we do
that for pytest too.

Adapt require_programs() to use subprocess directly rather than have to
solve the weird dependency chain of pmb.helpers.run which depends on the
global context being initialised before it's usable.

Signed-off-by: Caleb Connolly <caleb@postmarketos.org>