Fix clearcase diff tests.
Review Request #12603 — Created Sept. 14, 2022 and submitted
The diff tests had an incorrect return type for the directory diffing
command invocations (returning a single bytestring rather than a list of
bytes as expected by
split_lines=True. This happened to work correctly
because of the particular nature of the diff results, but wasn't
following the right code paths afterword. Fixing the return value
therefore required updating subsequent calls in the expect order.
Ran unit tests.