[gdb/testsuite] Fix gdb.base/index-cache.exp with cc-with-gdb-index

[gdb/testsuite] Fix gdb.base/index-cache.exp with cc-with-gdb-index

With target board cc-with-gdb-index we run into:
...
FAIL: gdb.base/index-cache.exp: couldn't remove files in temporary cache dir
...

The problem is that there are no files to remove, because the index cache
isn't used, as indicated by $expecting_index_cache_use.

Fix this by only FAILing when $expecting_index_cache_use.

Tested on x86_64-linux.

gdb/testsuite/ChangeLog:

2021-08-01  Tom de Vries  <tdevries@suse.de>

	* gdb.base/index-cache.exp: