tests: always test fetch of unreachable with v0

Some tests check that fetching an unreachable object fails, but protocol
v2 allows such fetches. Unset GIT_TEST_PROTOCOL_VERSION so that these
tests are always run using protocol v0.

Signed-off-by: Jonathan Tan <jonathantanmy@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>