tests: don't match blank output lines as regexes in unified tests
Currently, the following unified test will pass:
$ echo foo
A blank output line (a line containing just two spaces) will match any
output.
The patch modifies the unified test runner to ignore empty strings
strings when do regular expression matching.
adding a
updating to branch default
1 files updated, 0 files merged, 0 files removed, 0 files unresolved
% expect error, cloning not allowed
abort: authorization failed
requesting all changes
% serve errors
% expect error, pulling not allowed
abort: authorization failed
pulling from http://localhost:$HGPORT/
searching for changes
% serve errors