Let enable indexer and see if that helps to avoid sporadic fails.

Change-Id: Ie8d27e75b57aa2e0b787d85bc855f58141f658e8
Signed-off-by: Andrey Loskutov <loskutov@gmx.de>
Reviewed-on: https://git.eclipse.org/r/c/jdt/eclipse.jdt.core/+/183231
Tested-by: JDT Bot <jdt-bot@eclipse.org>