CHEF-3068: please
[CHEF-3068] Fix test failures and guard against nil source_line

MatchData#matches doesn't appear to exist.  Also, source_line
will be nil for dynamically defined resources.

The change in the regular expression is to account for the fact that
the format of the source_line changes between unix-like and Windows
platforms.
[CHEF-3068] Add tests for Chef::Resource#defined_at
[CHEF-3068] Add a comment explaining the intent of the regexp.
Merge branch 'CHEF-3068'