Rollup merge of #41678 - GuillaumeGomez:rustdoc-test-warnings, r=alexcrichton

Add option to display warnings in rustdoc

Part of #41574.

r? @alexcrichton

The output for this file:

```rust
/// ```
/// fn foo(x: u32) {}
///
/// foo(2);
/// let x = 1;
/// panic!();
/// ```
fn foo() {}

/// ```
/// fn foo(x: u32) {}
///
/// foo(2);
/// let x = 1;
/// ```
fn foo2() {}

/// ```
/// fn foo(x: u32) {}
///
/// foo(2);
/// let x = 1;
/// panic!();
/// ```
fn foo3() {}

fn main() {
}
```

is the following:

```
> ./build/x86_64-apple-darwin/stage1/bin/rustdoc -Z unstable-options --display-warnings --test test.rs

running 3 tests
test test.rs - foo (line 1) ... FAILED
test test.rs - foo3 (line 18) ... FAILED
test test.rs - foo2 (line 10) ... ok

successes:

---- test.rs - foo2 (line 10) stdout ----
	warning: unused variable: `x`
 --> <anon>:2:8
  |
2 | fn foo(x: u32) {}
  |        ^
  |
  = note: #[warn(unused_variables)] on by default

warning: unused variable: `x`
 --> <anon>:5:5
  |
5 | let x = 1;
  |     ^
  |
  = note: #[warn(unused_variables)] on by default

successes:
    test.rs - foo2 (line 10)

failures:

---- test.rs - foo (line 1) stdout ----
	warning: unused variable: `x`
 --> <anon>:2:8
  |
2 | fn foo(x: u32) {}
  |        ^
  |
  = note: #[warn(unused_variables)] on by default

warning: unused variable: `x`
 --> <anon>:5:5
  |
5 | let x = 1;
  |     ^
  |
  = note: #[warn(unused_variables)] on by default

thread 'rustc' panicked at 'test executable failed:

thread 'main' panicked at 'explicit panic', <anon>:6
note: Run with `RUST_BACKTRACE=1` for a backtrace.

', src/librustdoc/test.rs:317
note: Run with `RUST_BACKTRACE=1` for a backtrace.

---- test.rs - foo3 (line 18) stdout ----
	warning: unused variable: `x`
 --> <anon>:2:8
  |
2 | fn foo(x: u32) {}
  |        ^
  |
  = note: #[warn(unused_variables)] on by default

warning: unused variable: `x`
 --> <anon>:5:5
  |
5 | let x = 1;
  |     ^
  |
  = note: #[warn(unused_variables)] on by default

thread 'rustc' panicked at 'test executable failed:

thread 'main' panicked at 'explicit panic', <anon>:6
note: Run with `RUST_BACKTRACE=1` for a backtrace.

', src/librustdoc/test.rs:317

failures:
    test.rs - foo (line 1)
    test.rs - foo3 (line 18)

test result: FAILED. 1 passed; 2 failed; 0 ignored; 0 measured
```

0 files changed, 0 insertions, 0 deletions