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

test/syscalls/build_defs.bzl

@@ -4,7 +4,7 @@
 # on the host (native) and runsc.
 def syscall_test(
         test,
-        shard_count = 1,
+        shard_count = 5,
         size = "small",
         use_tmpfs = False,
         add_overlay = False,