[gdb/testsuite] Add wait-for-index-cache in gdb.dwarf2/per-bfd-sharing.exp

If we make writing an index-cache entry very slow by doing this in
index_cache::store:
...
   try
     {
+      sleep (15);
       index_cache_debug ("writing index cache for objfile %s",
 			 bfd_get_filename (per_bfd->obfd));
...
we run into:
...
FAIL: gdb.dwarf2/per-bfd-sharing.exp: \
  couldn't remove files in temporary cache dir
...

The FAIL happens because there is no index-cache entry in the cache dir.

The problem is that gdb is killed (by gdb_exit) before the index-cache entry
is written.

Fix this by using "maint wait-for-index-cache".

Tested on x86_64-linux.

PR testsuite/30528
Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=30528