gdb/testsuite: adjust ensure_gdb_index to cooked_index_functions::dump changes

Following 7d82b08e ("gdb/dwarf: dump cooked index contents in
cooked_index_functions::dump"), I see some failures like:

    (gdb) mt print objfiles with-mf^M
    ^M
    Object file /home/smarchi/build/binutils-gdb/gdb/testsuite/outputs/gdb.base/with-mf/with-mf:  Objfile at 0x614000005040, bfd at 0x6120000e08c0, 18 minsyms    ^M
    ^M
    Cooked index in use:^M
    ^M
    ...
    (gdb) FAIL: gdb.base/with-mf.exp: check if index present

This is because the format of the "Cooked index in use" line changed
slightly.  Adjust ensure_gdb_index to expect the trailing colon.

Change-Id: If0a87575c02d8a0bc0d4b8ead540c234c62760f8