Set random_test size to large

This test is regularly timing out on ci.bazel.io: http://ci.bazel.io/job/re2/BAZEL_VERSION=HEAD,PLATFORM_NAME=linux-x86_64/36/console.
Increasing it size should make that test pass 100% of the time.

Change-Id: I30a7fa31e4d800533d957394940c53f980858695
Reviewed-on: https://code-review.googlesource.com/4010
Reviewed-by: Paul Wankadia <junyer@google.com>
2 files changed