feat: expose test client under testing feature

[tisonkun]

Motivation

There is a crate axum-test-helper for the same purpose and it seems widely used. Since the crate is less actively maintained, I suppose we can implement the same on the upstream so keep it updated.

Solution

#[cfg(any(test, feature = "testing"))]
pub mod test_helpers;

... and export all inner methods for test clients.

@jplatte

[tisonkun]

Some compile errors happen. I may spend more time investigating, while the "expected outer doc comment" seems a false-positive.

Anyway, I'd like to hear from the maintainer whether this is a good way to go first.

[jplatte]

This was previously rejected in #1146, sorry.

[tisonkun]

Thanks for your information!

I just noticed that it's a thin wrapper of reqwest client so that I should be able to bundle it in my software :D