test_runner: Fix type annotation of get_tests_from_suite.
test_runner: Fix type annotation of get_tests_from_suite.

It is really a generator of test cases from the test suite. Which should
be typed as an `Iterable` instead.

Signed-off-by: Zixuan James Li <74923aee05c23ca90566381b5d6b0cd9451b6054@gmail.com>

zulip/zulip,andersk/zulip,rht/zulip,rht/zulip,andersk/zulip,andersk/zulip,zulip/zulip,zulip/zulip,rht/zulip,andersk/zulip,zulip/zulip,andersk/zulip,zulip/zulip,andersk/zulip,rht/zulip,andersk/zulip,zulip/zulip,rht/zulip,rht/zulip,rht/zulip,zulip/zulip