"Error flowing watermark" in operator tests.

[Sukrit Kalra]

The master prints an error when thetest_input_receiver_join is run, but the test still succeeds and does not fail.

Example

Run the following command: cargo test test_input_receiver_join -- --nocapture. The log is:

running 1 test
Mar 21 16:02:58.951 DEBG Starting node 0
Mar 21 16:02:58.953 DEBG Executing operator 97944689-ce67-a82c-c506-8ba4659991b6 on node 0
Mar 21 16:02:58.953 DEBG Executing operator a1a1bc90-4811-2c4b-0580-1b8b3546c0ab on node 0
Mar 21 16:02:58.954 DEBG Executing operator 96e1c913-b9c0-12da-3817-0fa1349bebd0 on node 0
Mar 21 16:02:58.955 DEBG Notfiying node initialized
Error flowing watermark
test test_input_receiver_join ... ok

test result: ok. 1 passed; 0 failed; 0 ignored; 0 measured; 1 filtered out

As far as I can tell, the regression was introduced in e24a859.