iotests/testrunner.py: refactor test_field_width

A lot of Optional[] types doesn't make code beautiful.
test_field_width defaults to 8, but that is never used in the code.

More over, if we want some default behavior for single call of
test_run(), it should just print the whole test name, not limiting or
expanding its width, so 8 is bad default.

So, just drop the default as unused for now.

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Message-Id: <20211210201450.101576-1-vsementsov@virtuozzo.com>
Reviewed-by: John Snow <jsnow@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>