Add //java/... to Protobuf pipeline (#950)

Java-specific targets were moved into //java in protocolbuffers/protobuf#7190

Author: Yannic

buildkite/pipelines/protobuf-postsubmit.yml

@@ -3,20 +3,23 @@ platforms:
   ubuntu1604:
     test_targets:
       - "//:all"
+      - "//java/..."
       # `cc_proto_blacklist_test` only works as `@com_google_protobuf//:cc_proto_blacklist_test`.
       # https://github.com/bazelbuild/bazel/issues/10590
       - "-//:cc_proto_blacklist_test"
       - "@com_google_protobuf//:cc_proto_blacklist_test"
   macos:
     test_targets:
       - "//:all"
+      - "//java/..."
       # `cc_proto_blacklist_test` only works as `@com_google_protobuf//:cc_proto_blacklist_test`.
       # https://github.com/bazelbuild/bazel/issues/10590
       - "-//:cc_proto_blacklist_test"
       - "@com_google_protobuf//:cc_proto_blacklist_test"
   windows:
     test_targets:
       - "//:all"
+      - "//java/..."
       # `cc_proto_blacklist_test` only works as `@com_google_protobuf//:cc_proto_blacklist_test`.
       # https://github.com/bazelbuild/bazel/issues/10590
       - "-//:cc_proto_blacklist_test"