tests: handle case in test-remotefilelog-clone.t where output is out of order
test-remotefilelog-clone.t is flaky, and fails sporadically as follows:
$ hg clone --noupdate ssh://user@dummy/shallow full streaming all changes + abort: unexpected response from remote server: empty string remote: abort: Cannot clone from a shallow repo to a full repo. - abort: unexpected response from remote server: empty string 
The issue happens because the line starting "abort" and the line starting
"remote: abort" come from different processes, and there's no way to
guarantee the order in which they are output.
This change attempts to address the issue by capturing the output of the
local process and examining it separately.
Differential Revision: https://phab.mercurial-scm.org/D1279