infra/common.sh: add shard_should_run()
infra/common.sh: add shard_should_run()

Detects whether test block should be run in the current test shard.

if shard_should_run; then
 ...
fi

Bug: b/185520507
Change-Id: I408d226651630a01c5e447dbff7d26b73426ce42

webmproject/libwebp,PKRoma/libwebp,webmproject/libwebp,PKRoma/libwebp,webmproject/libwebp,PKRoma/libwebp,webmproject/libwebp,PKRoma/libwebp,PKRoma/libwebp,webmproject/libwebp,PKRoma/libwebp