tests: odb: use correct OID length

The `search_object` function takes the OID length as one of its
parameters, where its maximum length is `GIT_OID_HEXSZ`. The `exists`
function of the fake backend used `GIT_OID_RAWSZ` though, leading to
only the first half of the OID being used when finding the correct
object.

tests/odb/backend/backend_helpers.c

diff --git a/tests/odb/backend/backend_helpers.c b/tests/odb/backend/backend_helpers.c
index 3317136..f9211e7 100644
--- a/tests/odb/backend/backend_helpers.c
+++ b/tests/odb/backend/backend_helpers.c
@@ -34,7 +34,7 @@
 
 	fake->exists_calls++;
 
-	return search_object(NULL, fake, oid, GIT_OID_RAWSZ) == GIT_OK;
+	return search_object(NULL, fake, oid, GIT_OID_HEXSZ) == GIT_OK;
 }
 
 static int fake_backend__read(