test should_check_status_of_request_when_its_resolved fails spuriously

[ordian]

See e.g. https://gitlab.parity.io/parity/parity-ethereum/-/jobs/169369

failures:

---- v1::tests::mocked::signing::should_check_status_of_request_when_its_resolved stdout ----
thread 'v1::tests::mocked::signing::should_check_status_of_request_when_its_resolved' panicked at 'assertion failed: `(left == right)` (left: `Some("{\"jsonrpc\":\"2.0\",\"result\":null,\"id\":1}")`, right: `Some("{\"jsonrpc\":\"2.0\",\"result\":\"0x0000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000001\",\"id\":1}")`, diff: ` Some("{\"jsonrpc\":\"2.0\",\"result\":null,\"id\":1}") Some("{\"jsonrpc\":\"2.0\",\"result\":\"0x0000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000001\",\"id\":1}") `)', rpc/src/v1/tests/mocked/signing.rs:235:2
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.
stack backtrace:
   0: std::sys::unix::backtrace::tracing::imp::unwind_backtrace
             at src/libstd/sys/unix/backtrace/tracing/gcc_s.rs:39
   1: std::sys_common::backtrace::_print
             at src/libstd/sys_common/backtrace.rs:71
   2: std::panicking::default_hook::{{closure}}
             at src/libstd/sys_common/backtrace.rs:59
             at src/libstd/panicking.rs:197
   3: std::panicking::default_hook
             at src/libstd/panicking.rs:208
   4: std::panicking::rust_panic_with_hook
             at src/libstd/panicking.rs:474
   5: std::panicking::continue_panic_fmt
             at src/libstd/panicking.rs:381
   6: std::panicking::begin_panic_fmt
             at src/libstd/panicking.rs:336
   7: parity_rpc::v1::tests::mocked::signing::should_check_status_of_request_when_its_resolved
             at rpc/src/v1/tests/mocked/signing.rs:235
   8: <alloc::boxed::Box<F> as core::ops::function::FnOnce<A>>::call_once
             at /rustc/3c235d5600393dfe6c36eeed34042efad8d4f26e/src/liballoc/boxed.rs:702
   9: __rust_maybe_catch_panic
             at src/libpanic_unwind/lib.rs:87
  10: test::run_test::run_test_inner::{{closure}}
             at /rustc/3c235d5600393dfe6c36eeed34042efad8d4f26e/src/libstd/panicking.rs:272
             at /rustc/3c235d5600393dfe6c36eeed34042efad8d4f26e/src/libstd/panic.rs:388
             at src/libtest/lib.rs:1468