tests: add tests for the mempack ODB backend

Our mempack ODB backend has no test coverage at all right now. Add a
simple test suite to at least have some coverage of the most basic
operations on the ODB.

tests/odb/backend/mempack.c

+#include "clar_libgit2.h"
+#include "repository.h"
+#include "backend_helpers.h"
+#include "git2/sys/mempack.h"
+
+static git_odb *_odb;
+static git_oid _oid;
+static git_odb_object *_obj;
+static git_repository *_repo;
+
+void test_odb_backend_mempack__initialize(void)
+{
+	git_odb_backend *backend;
+
+	cl_git_pass(git_mempack_new(&backend));
+	cl_git_pass(git_odb_new(&_odb));
+	cl_git_pass(git_odb_add_backend(_odb, backend, 10));
+	cl_git_pass(git_repository_wrap_odb(&_repo, _odb));
+}
+
+void test_odb_backend_mempack__cleanup(void)
+{
+	git_odb_object_free(_obj);
+	git_odb_free(_odb);
+	git_repository_free(_repo);
+}
+
+void test_odb_backend_mempack__write_succeeds(void)
+{
+	const char *data = "data";
+	cl_git_pass(git_odb_write(&_oid, _odb, data, strlen(data) + 1, GIT_OBJ_BLOB));
+	cl_git_pass(git_odb_read(&_obj, _odb, &_oid));
+}
+
+void test_odb_backend_mempack__read_of_missing_object_fails(void)
+{
+	cl_git_pass(git_oid_fromstr(&_oid, "f6ea0495187600e7b2288c8ac19c5886383a4633"));
+	cl_git_fail_with(GIT_ENOTFOUND, git_odb_read(&_obj, _odb, &_oid));
+}
+
+void test_odb_backend_mempack__exists_of_missing_object_fails(void)
+{
+	cl_git_pass(git_oid_fromstr(&_oid, "f6ea0495187600e7b2288c8ac19c5886383a4633"));
+	cl_assert(git_odb_exists(_odb, &_oid) == 0);
+}
+
+void test_odb_backend_mempack__exists_with_existing_objects_succeeds(void)
+{
+	const char *data = "data";
+	cl_git_pass(git_odb_write(&_oid, _odb, data, strlen(data) + 1, GIT_OBJ_BLOB));
+	cl_assert(git_odb_exists(_odb, &_oid) == 1);
+}