iotests.py: Assume a couple of variables as given

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>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Reviewed-by: Willian Rampazzo <willianr@redhat.com>
Message-Id: <20210118105720.14824-2-mreitz@redhat.com>

tests/qemu-iotests/300

@@ -28,7 +28,6 @@ import qemu
 
 BlockBitmapMapping = List[Dict[str, Union[str, List[Dict[str, str]]]]]
 
-assert iotests.sock_dir is not None
 mig_sock = os.path.join(iotests.sock_dir, 'mig_sock')