test-dispatch: stabilize the test
When cwd is removed and `hg` is executed, some shells may run `getcwd`
before forking and executing, some may not do it, some may print a
different error message.
The test should be shell-independent so let's just avoid checking the error
message.
Differential Revision: https://phab.mercurial-scm.org/D1282
{
"conduit_uri": "https://phab.mercurial-scm.org/api",
"repository.callsign": "HG",
"arc.land.onto.default": "@",
"base": "hg:.^"
}