test: set shard_count to 5 by default
test: set shard_count to 5 by default

In cl/264434674 and cl/264498919, we stop running test cases
in parallel to not overload test hosts. But now tests requires
more time to run, so we need to increase a default number of
shards or a default test timeout. Let's start with increasing
the number of shards and see how it will works.

PiperOrigin-RevId: 264917055

google/gvisor,google/gvisor,google/gvisor,google/gvisor,google/gvisor,google/gvisor,google/gvisor,google/gvisor