xds_client_test: shard test to avoid timeouts (#31652)

* xds_client_test: increase test size so it gets a longer timeout

* shard test instead of increasing size

* Automated change: Fix sanity tests

Co-authored-by: markdroth <markdroth@users.noreply.github.com>

test/core/xds/BUILD

@@ -147,6 +147,7 @@ grpc_cc_test(
     srcs = ["xds_client_test.cc"],
     external_deps = ["gtest"],
     language = "C++",
+    shard_count = 10,
     uses_event_engine = True,
     uses_polling = False,
     deps = [