[gdb/testsuite] Move "maint wait-for-index-cache" ALAP in gdb.base/index-cache.exp

[gdb/testsuite] Move "maint wait-for-index-cache" ALAP in gdb.base/index-cache.exp

In test-case gdb.base/index-cache.exp proc run_test_with_flags contains:
...
	clean_restart ${testfile}

	# The tests generally want to check the cache, so make sure it
	# has completed its work.
	gdb_test_no_output "maintenance wait-for-index-cache"
...

This however hides data races between:
- index-cache writing (due to file $exec), and
- symbol lookups (due to subsequent ptype commands).

Fix this by:
- moving the "maintenance wait-for-index-cache" to proc check_cache_stats, and
- moving all calls to proc check_cache_stats ALAP.

Tested on x86_64-linux.