Move _FakeCurl from tests/ganeti.rapi.client to ganeti.rapi.testutils

This is preparation for the mock system, where we need the same cURL
mock.

Signed-off-by: René Nussbaumer <rn@google.com>
Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: René Nussbaumer <rn@google.com>