[gdb/testsuite] Require cooked index in two test-cases

After running the testsuite with target board cc-with-gdb-index I ran found
failures in test-cases:
- gdb.dwarf2/backward-spec-inter-cu.exp
- gdb.dwarf2/forward-spec-inter-cu.exp

Fix this by requiring a cooked index.

Tested on x86_64-linux.