tests: do not reuse OID from backend

In order to make the fake backend more useful, we want to enable it
holding multiple object references. To do so, we need to decouple it
from the single fake OID it currently holds, which we simply move up
into the calling tests.
diff --git a/tests/odb/backend/nonrefreshing.c b/tests/odb/backend/nonrefreshing.c
index efc285a..42da312 100644
--- a/tests/odb/backend/nonrefreshing.c
+++ b/tests/odb/backend/nonrefreshing.c
@@ -8,6 +8,9 @@
 #define HASH "deadbeefdeadbeefdeadbeefdeadbeefdeadbeef"
 #define EMPTY_HASH "e69de29bb2d1d6434b8b29ae775ad8c2e48c5391"
 
+static git_oid _oid;
+static git_oid _empty_oid;
+
 static void setup_repository_and_backend(git_error_code error_code, const char *hash)
 {
 	git_odb *odb = NULL;
@@ -25,6 +28,12 @@
 	_fake = (fake_backend *)backend;
 }
 
+void test_odb_backend_nonrefreshing__initialize(void)
+{
+	git_oid_fromstr(&_oid, HASH);
+	git_oid_fromstr(&_empty_oid, EMPTY_HASH);
+}
+
 void test_odb_backend_nonrefreshing__cleanup(void)
 {
 	cl_git_sandbox_cleanup();
@@ -37,7 +46,7 @@
 	setup_repository_and_backend(GIT_ENOTFOUND, HASH);
 
 	cl_git_pass(git_repository_odb__weakptr(&odb, _repo));
-	cl_assert_equal_b(false, git_odb_exists(odb, &_fake->oid));
+	cl_assert_equal_b(false, git_odb_exists(odb, &_oid));
 
 	cl_assert_equal_i(1, _fake->exists_calls);
 }
@@ -49,7 +58,7 @@
 	setup_repository_and_backend(GIT_ENOTFOUND, HASH);
 
 	cl_git_fail_with(
-		git_object_lookup(&obj, _repo, &_fake->oid, GIT_OBJ_ANY),
+		git_object_lookup(&obj, _repo, &_oid, GIT_OBJ_ANY),
 		GIT_ENOTFOUND);
 
 	cl_assert_equal_i(1, _fake->read_calls);
@@ -62,7 +71,7 @@
 	setup_repository_and_backend(GIT_ENOTFOUND, HASH);
 
 	cl_git_fail_with(
-		git_object_lookup_prefix(&obj, _repo, &_fake->oid, 7, GIT_OBJ_ANY),
+		git_object_lookup_prefix(&obj, _repo, &_oid, 7, GIT_OBJ_ANY),
 		GIT_ENOTFOUND);
 
 	cl_assert_equal_i(1, _fake->read_prefix_calls);
@@ -79,7 +88,7 @@
 	cl_git_pass(git_repository_odb__weakptr(&odb, _repo));
 
 	cl_git_fail_with(
-		git_odb_read_header(&len, &type, odb, &_fake->oid),
+		git_odb_read_header(&len, &type, odb, &_oid),
 		GIT_ENOTFOUND);
 
 	cl_assert_equal_i(1, _fake->read_header_calls);
@@ -92,7 +101,7 @@
 	setup_repository_and_backend(GIT_OK, HASH);
 
 	cl_git_pass(git_repository_odb__weakptr(&odb, _repo));
-	cl_assert_equal_b(true, git_odb_exists(odb, &_fake->oid));
+	cl_assert_equal_b(true, git_odb_exists(odb, &_oid));
 
 	cl_assert_equal_i(1, _fake->exists_calls);
 }
@@ -103,7 +112,7 @@
 
 	setup_repository_and_backend(GIT_OK, EMPTY_HASH);
 
-	cl_git_pass(git_object_lookup(&obj, _repo, &_fake->oid, GIT_OBJ_ANY));
+	cl_git_pass(git_object_lookup(&obj, _repo, &_empty_oid, GIT_OBJ_ANY));
 
 	cl_assert_equal_i(1, _fake->read_calls);
 
@@ -116,7 +125,7 @@
 
 	setup_repository_and_backend(GIT_OK, EMPTY_HASH);
 
-	cl_git_pass(git_object_lookup_prefix(&obj, _repo, &_fake->oid, 7, GIT_OBJ_ANY));
+	cl_git_pass(git_object_lookup_prefix(&obj, _repo, &_empty_oid, 7, GIT_OBJ_ANY));
 
 	cl_assert_equal_i(1, _fake->read_prefix_calls);
 
@@ -133,7 +142,7 @@
 
 	cl_git_pass(git_repository_odb__weakptr(&odb, _repo));
 
-	cl_git_pass(git_odb_read_header(&len, &type, odb, &_fake->oid));
+	cl_git_pass(git_odb_read_header(&len, &type, odb, &_oid));
 
 	cl_assert_equal_i(1, _fake->read_header_calls);
 }