tests: add checks for dumpstr output into read-write test

tests: add checks for dumpstr output into read-write test

util.c:dumpstr() now aligns offsets in accordance with the total length
of the dump and also may perform only partial dump, so let's add checks
for these cases.

* tests/fill_memory.c (fill_memory_ex): Change the type of the period
argument from unsigned char to unsigned int.
* tests/tests.h (fill_memory_ex): Likewise.
* tests/read-write.c (dump_str_ex): Rename from dump_str, add idx_w
argument, support len greater than 240 bytes.
(dump_str): New function, wrapper for dump_str_ex with the default index
width.
(print_hex): Add checks for dumpstr output, close fd 1 later.