[gdb/testsuite] Add gdb.dwarf2/dwz-unused-pu.exp

Add a new test-case gdb.dwarf2/dwz-unused-pu.exp that checks that a symbol
from an unused PU is not accessible.

Passes with the relevant target boards:
- unix (using the cooked index),
- readnow (using no index at all),
- cc-with-gdb-index (using .gdb_index), and
- cc-with-debug-names (using .debug_names).

Tested on x86_64-linux.