tests: put mocking functionality into a crate (tokio-rs#2009)

... instead of `#[path = ""]` importing it everywhere.

Make sure to use a diff review tool that understands file renaming 😅
(GitHub's diff view does.)

Transparency: I want to use the mocking functionality in the development
of a tracing component out-of-tree.

Additionally, this reduces the use of `#[path] mod` and file
multiple-inclusion, which aren't that great of a practice.

The tracing test support module was already well self-contained, due to
being `#[path] mod` used in multiple places. As such, extracting it to
its own crate is rather mechanical, with no surprising blockers.

We additionally move the tracing-futures support module contents into
tracing_mock, for convenience. The one function which relies on
tokio-test is made optional.

It's a reasonable result for this functionality to stay unpublished, and
only used inside the repo, but pulling it out into a directly reusable
chunk instead of abusing the module system to reuse it via
multiple-inclusion is an improvement to code structure and
modularization.

tracing-attributes/tests/ret.rs

@@ -1,5 +1,6 @@
 use std::convert::TryFrom;
 use std::num::TryFromIntError;
+use tracing_mock::*;
 
 use tracing::{collect::with_default, Level};
 use tracing_attributes::instrument;

tracing-subscriber/tests/support.rs

@@ -1,20 +1,15 @@
 #![allow(missing_docs, dead_code)]
-pub use self::support::{event, field, span, subscriber};
-// This has to have the same name as the module in `tracing`.
-// path attribute requires referenced module to have same name so allow module inception here
-#[allow(clippy::module_inception)]
-#[path = "../../tracing/tests/support/mod.rs"]
-mod support;
-
-use self::{
-    event::MockEvent,
-    span::{MockSpan, NewSpan},
-    subscriber::{Expect, MockHandle},
-};
+pub use tracing_mock::{event, field, span, subscriber};
+
 use tracing_core::{
     span::{Attributes, Id, Record},
     Event, Subscriber,
 };
+use tracing_mock::{
+    event::MockEvent,
+    span::{MockSpan, NewSpan},
+    subscriber::{Expect, MockHandle},
+};
 use tracing_subscriber::{
     layer::{Context, Layer},
     registry::{LookupSpan, SpanRef},

tracing/tests/no_subscriber.rs

@@ -2,13 +2,11 @@
 
 use tracing::subscriber::{self, NoSubscriber};
 
-mod support;
-
 #[cfg_attr(target_arch = "wasm32", wasm_bindgen_test::wasm_bindgen_test)]
 #[test]
 fn no_subscriber_disables_global() {
     // Reproduces https://github.com/tokio-rs/tracing/issues/1999
-    let (subscriber, handle) = support::subscriber::mock().done().run_with_handle();
+    let (subscriber, handle) = tracing_mock::subscriber::mock().done().run_with_handle();
     subscriber::set_global_default(subscriber).expect("setting global default must succeed");
     subscriber::with_default(NoSubscriber::default(), || {
         tracing::info!("this should not be recorded");

tracing/tests/subscriber.rs

@@ -12,9 +12,7 @@ use tracing::{
     Event, Level, Metadata,
 };
 
-mod support;
-
-use self::support::*;
+use tracing_mock::*;
 
 #[cfg_attr(target_arch = "wasm32", wasm_bindgen_test::wasm_bindgen_test)]
 #[test]