extend scope of testing in gdb.ada/arrayptr

extend scope of testing in gdb.ada/arrayptr

We add testing of taking a slice of an array access.  And we also
introduce the same amount of testing, but with an access to a
constrained array.

gdb/testsuite/ChangeLog:

        * gdb.ada/arrayptr/foo.adb: Add access to constrained array.
        * gdb.ada/arrayptr.exp: Add new tests.