There are a couple of environment variables that we fetch with
os.environ.get() without supplying a default. Clearly they are required
and expected to be set by the ./check script (as evidenced by
execute_setup_common(), which checks for test_dir and
qemu_default_machine to be set, and aborts if they are not).
Using .get() this way has the disadvantage of returning an Optional[str]
type, which mypy will complain about when tests just assume these values
to be str.
Use [] instead, which raises a KeyError for environment variables that
are not set. When this exception is raised, catch it and move the abort
code from execute_setup_common() there.
Drop the 'assert iotests.sock_dir is not None' from iotest 300, because
that sort of thing is precisely what this patch wants to prevent.
Signed-off-by: Max Reitz<mreitz@redhat.com>