[gdb/testsuite] Simplify gdb.dwarf2/forward-spec.exp

Test-case gdb.dwarf2/forward-spec.exp contains a non-trivial gdb_test_multiple
to parse this cooked_index_entry:
...
    [5] ((cooked_index_entry *) 0x7f01f0004040)^M
    name:       v^M
    canonical:  v^M
    qualified:  ns::v^M
    DWARF tag:  DW_TAG_variable^M
    flags:      0x2 [IS_STATIC]^M
    DIE offset: 0xcb^M
    parent:     ((cooked_index_entry *) 0x7f01f00040a0) [ns]^M
...
which allows us to verify that the entry has a parent.

After commit 8f258a6c ("[gdb/symtab] Dump qualified name of
cooked_index_entry") that's no longer necessary.

Simplify this by checking for ns::v instead.

While we're at it, also fix the test-case for target boards readnow,
cc-with-gdb-index and cc-with-debug-names.

Tested on x86_64-linux.