Unify the WatchLog.wait_for_line/s() API

Unify the WatchLog.wait_for_line/s() API

Rather than using two distinct functions for matching either one pattern
(wait_for_line()), or any of multiple patterns (wait_for_lines()), use a
single function that handles both in the same way.

Extend the wait_for_line() API:
1. To allow for usage of one or more FlexPatterns, i.e. either plain
   strings to be matched verbatim, or regular expressions. Both can be
   used interchangeably to provide the caller to write simple and
   readable test code, while allowing for increased complexity to allow
   special cases.
2. Always return the regex match, which allows the caller to identify
   which line was matched, as well as to extract any additional
   information, such as individual regex groups.