test: Helpful panic messages for tokio_test::io

[vi]

Introduce tokio_test::io::Builder::name to configure name of the mock object, to include in panic messages.

Also show number of remaining actions or action index in some cases to help debugging failed tests.

Motivation

When using tokio_test::io::Mock, it is tricky to debug problems, especially if multiple mocks are used. For example, if it panics with "unexpected write", it is hard to determine which of the mock objects faced the unexpected write and at what position of the scenario.

Solution

Builder gained name method to set make Mock remember a string.

Panic messages are adjusted to include that name (if set), also some additional information (like the number of remaining actions) to aid hack-try-print-repeat-based debugging sessions.

Before:

unexpected write

After:

unexpected write (name my_mock_name, 0 actions remain)

[mox692]

Looks good to me!

[vi]

Shall I submit further additions to io::Mock (e.g. ability to ignore unexpected things after certain points or explicitly inject EOF in reading part while still expecting more writes to come) or it would better suit in a separate crate?

[mox692]

If that feature can be implemented outside of tokio, then it would generally be better to implement it elsewhere.