tests: setup dummyssh as the default ssh
To significantly reduce boilerplate in tests. One test is updated to
show that it works, I expect to do the rest in follow up commits.
Differential Revision: https://phab.mercurial-scm.org/D11244
#if no-windows no-osx
$ mkdir -p xdgconf/hg
$ echo '[ui]' > xdgconf/hg/hgrc
$ echo 'username = foobar' >> xdgconf/hg/hgrc
$ XDG_CONFIG_HOME="`pwd`/xdgconf" ; export XDG_CONFIG_HOME
$ unset HGRCPATH
$ hg config ui.username 2>/dev/null
foobar
#endif