Infra: Use smaller batches for GLES1 tests The GLES1 conformance tests are few, but each take a long time. As such, --bot-mode only divides them by platform and otherwise the tests run serially. This change uses a smaller batch size to increase parallelism. Bug: angleproject:6644 Change-Id: I1401db0899d4f57735cbfdfb4b051a2d19f8e2a5 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/3933829 Reviewed-by: Jamie Madill <jmadill@chromium.org> Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>