mdbx-test: minor extension for jitter test case.

Trivial test for https://github.com/erthink/libmdbx/issues/146.

Change-Id: If5b365ebedf8609c9ec12569d5e5173799379195

test/test.h

@@ -287,6 +287,9 @@ public:
 };
 
 class testcase_jitter : public testcase {
+protected:
+  void check_dbi_error(int expect, const char *stage);
+
 public:
   testcase_jitter(const actor_config &config, const mdbx_pid_t pid)
       : testcase(config, pid) {}