Refactor: cts: Be more explicit about converting ExitStatus.

There are platform-specific quirks to this code. Different versions of
python handle converting enums to ints and strings differently when
f-strings are involved. I unfortunately haven't been keeping track of
what happens where to write better comments.

So instead, just explicitly do all the conversions in _log_end_test
instead of hoping the implicit conversions will do the right thing.