[testsuite/Ada] Add testing of access to packed arrays.

[testsuite/Ada] Add testing of access to packed arrays.

This patch adds some tests that evidence a regression fixed by
the following patch from Pedro Alves:

    [PATCH] ada-lang.c:coerce_unspec_val_to_type: Preserve laziness
    http://www.sourceware.org/ml/gdb-patches/2013-07/msg00178.html

gdb/testsuite/ChangeLog:

        * gdb.ada/arrayptr/foo.adb: Add some code defining an access
        to a packed array.
        * gdb.ada/arrayptr.exp: Add a few tests using that new access
        to packed array.