RE2 differs from Ruby in handling multiple-line strings. The string "foo\n"
will not match the regular expression "foo$" unless multi-line mode is enabled
(and it's off by default).
Since we're already scanning the build trace line by line (and so multi-line
coverage regular expressions won't work), we can fix this by removing the line
separator before scanning the string.
Given a valid pipeline job, and a regex which wouldn't match to a jobs
trace, the stream of the trace would return the File object. This was
not the case when it matched a value, as that would have been return
from the block.
Now the `extract_coverage` method returns `nil` if no match was found.
