Simplify gdb.base/memops-watchpoint.exp

gdb.base/memops-watchpoint.exp has the same code basically repeated
three times.  Simplify by refactoring the common code to a procedure.

Modernize the gdb_test_multiple call a little bit while at it: use
-wrap and $gdb_test_name.

Change-Id: I42d9cbccf32760a2bc3dab2aa60412ad5a2ae291