utils/ssh: test fails if host lookup succeeds

[Roger Peppe]

TestClientNoKeys uses "test.invalid" for a host name. The test fails (after a long time)
because "test.invalid" resolves to an IP address for me, so I see:

FAIL: ssh_gocrypto_test.go:101: SSHGoCryptoCommandSuite.TestClientNoKeys

[LOG] 36.85478 DEBUG juju.utils.ssh command.Wait finished: signal: killed
ssh_gocrypto_test.go:113:
    // error message differs based on whether using cgo or not
    c.Assert(err, gc.ErrorMatches, "(dial tcp: )?lookup test.invalid: no such host")
... error string = "dial tcp 92.242.132.16:22: connection timed out"
... regex string = "(dial tcp: )?lookup test.invalid: no such host"

No test should rely on external DNS to provide a reliable resolution.
"0.1.2.3" is more reliable, as it's invalid at a much lower level.