dbs-upcall: Need to fix unit test failure.

[wllenyj]

03:09 [130] $ cargo test -p dbs-upcall -- test_upcall_client_send_request                         
    Finished test [unoptimized + debuginfo] target(s) in 0.05s                                    
     Running unittests (target/debug/deps/dbs_upcall-be6e9a9c8aea1888)

running 3 tests                                                                                   
test tests::test_upcall_client_send_request_error ... ok                                          
test tests::test_upcall_client_send_request ... FAILED                                            
test tests::test_upcall_client_send_request_without_result ... FAILED

failures:                                                                                         

---- tests::test_upcall_client_send_request stdout ----                                           
thread 'tests::test_upcall_client_send_request' panicked at 'called `Result::unwrap()` on an `Err` value: Kind(WouldBlock)', crates/dbs-upcall/src/lib.rs:698:51
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace                     

---- tests::test_upcall_client_send_request_without_result stdout ----                            
thread 'tests::test_upcall_client_send_request_without_result' panicked at 'called `Result::unwrap()` on an `Err` value: Kind(WouldBlock)', crates/dbs-upcall/src/lib.rs:730:51

failures:                                                                                         
    tests::test_upcall_client_send_request                                                        
    tests::test_upcall_client_send_request_without_result                                         

test result: FAILED. 1 passed; 2 failed; 0 ignored; 0 measured; 17 filtered out; finished in 0.00s

[wllenyj]

Fixed in #167.