Document #[test_case] and #![test_runner]

Author: djrenren

src/doc/unstable-book/src/language-features/custom-test-frameworks.md

@@ -0,0 +1,33 @@
+# `custom_test_frameworks`
+
+The tracking issue for this feature is: [#50297]
+
+[#50297]: https://github.com/rust-lang/rust/issues/50297
+
+------------------------
+
+The `custom_test_frameworks` feature allows the use of `#[test_case]` and `#![test_runner]`.
+Any function, const, or static can be annotated with `#[test_case]` causing it to be aggregated (like `#[test]`)
+and be passed to the test runner determined by the `#![test_runner]` crate attribute.
+
+```rust
+#![feature(custom_test_frameworks)]
+#![test_runner(my_runner)]
+
+fn my_runner(tests: &[&i32]) {
+    for t in tests {
+        if **t == 0 {
+            println!("PASSED");
+        } else {
+            println!("FAILED");
+        }
+    }
+}
+
+#[test_case]
+const WILL_PASS: i32 = 0;
+
+#[test_case]
+const WILL_FAIL: i32 = 4;
+```
+

src/test/ui/issues/issue-11692-2.rs

@@ -10,5 +10,5 @@
 
 fn main() {
     concat!(test!());
-    //~^ error: `test` can only be used in attributes
+    //~^ error: cannot find macro `test!` in this scope
 }

src/test/ui/issues/issue-11692-2.stderr

@@ -1,4 +1,4 @@
-error: `test` can only be used in attributes
+error: cannot find macro `test!` in this scope
   --> $DIR/issue-11692-2.rs:12:13
    |
 LL |     concat!(test!());

src/test/ui/test-shadowing/auxiliary/test_macro.rs

@@ -11,4 +11,4 @@
 #[macro_export]
 macro_rules! test {
     () => {};
-}
+}

src/test/ui/test-shadowing/test-cant-be-shadowed.rs

@@ -15,4 +15,4 @@
 #[macro_use] extern crate test_macro;
 
 #[test]
-fn foo(){}
+fn foo(){}