run-tests: fix _checkhglib() so it's correct when using --with-hg.
Since this makes the warning less likely, upgrade it to a "real"
warning to stderr.
200 Script output follows
content-type: text/plain
content-length: 157
content-disposition: inline; filename="some \"text\".txt"
This is just some random text
that will go inside the file and take a few lines.
It is very boring to read, but computers don't
care about things like that.
host - - [date] "GET /?f=a23bf1310f6e;file=sub/some%20%22text%22.txt;style=raw HTTP/1.1" 200 -