gdb/testsuite: fix duplicate test names in gdb.base/index-cache.exp

gdb/testsuite: fix duplicate test names in gdb.base/index-cache.exp

Fix:

    DUPLICATE: gdb.base/index-cache.exp: test_cache_disabled: no files were created
    DUPLICATE: gdb.base/index-cache.exp: test_cache_disabled: check index-cache stats

We use `proc_with_prefix` for test_cache_disabled, but we call it twice.  So we
need an additional prefix to identify the specific call.  This patch adds that.

gdb/testsuite/ChangeLog:

	* gdb.base/index-cache.exp (test_cache_disabled): Add test_prefix
	parameter, update callers.

Change-Id: Idf382fd380c77a654e8a7aa236af50b65c96b1d2