gdb/testsuite: Test the effect of amdgpu-precise memory

The gdb.rocm/precise-memory.exp test currently checks that the "amdgpu
precise-memory" setting can be set.  It does not test that this setting
has any meaningful effect.

This patch extends this test to ensure that precise-memory has the
expected behaviour.

Change-Id: I58f72a51a566f04fc89114b94ee656c2e7ac35bb
Approved-by: Pedro Alves <pedro@palves.net>