tests: define GIT_TEST_SIDEBAND_ALL

tests: define GIT_TEST_SIDEBAND_ALL

Define a GIT_TEST_SIDEBAND_ALL environment variable meant to be used
from tests. When set to true, this overrides uploadpack.allowsidebandall
to true, allowing the entire test suite to be run as if this
configuration is in place for all repositories.

As of this patch, all tests pass whether GIT_TEST_SIDEBAND_ALL is unset
or set to 1.

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