journal: add the "repeating sequence" test case

journal: add the "repeating sequence" test case

This was the case that caused various problems that were fixed in
preceding patches, so it is good to add a test that uses it directly.

In "may_fail" test cases try again with a bigger buffer.

Instead of allocating various buffers on the stack, malloc them.
This is more reliable in case of big buffers, and allows tools like
valgrind and address sanitizer to find overflows more easily.