Fix mem region parsing regression and add test

Fix mem region parsing regression and add test

In my patch

  Get rid of VEC (mem_region)
  a664f67e50eff30198097d51cec0ec4690abb2a1

I introduced a regression, where the length of the memory region is
assigned to the "hi" field.  It should obviously be computed as "start +
length".  To my defense, no test had caught this :).  As a penance, I
wrote one.

gdb/ChangeLog:

	* Makefile.in (SUBDIR_UNITTESTS_SRCS): Add
	memory-map-selftests.c.
	(SUBDIR_UNITTESTS_OBS): Add memory-map-selftests.o.
	* memory-map.c (memory_map_start_memory): Fix computation of hi
	address.
	* unittests/memory-map-selftests.c: New file.