Rollup merge of #39597 - GuillaumeGomez:correct_rustdoc_test_file, r=alexcrichton

Display correct filename with --test option

Fixes #39592.

With the current files:

```rust
pub mod foo;

/// This is a Foo;
///
/// ```
/// println!("baaaaaar");
/// ```
pub struct Foo;

/// This is a Bar;
///
/// ```
/// println!("fooooo");
/// ```
pub struct Bar;
```

```rust
// note the whitespaces
/// ```
/// println!("foo");
/// ```
pub fn foo() {}
```

It displays:

```
./build/x86_64-apple-darwin/stage1/bin/rustdoc --test test.rs

running 3 tests
test test.rs - line 13 ... ok
test test.rs - line 5 ... ok
test foo.rs - line 2 ... ok

test result: ok. 3 passed; 0 failed; 0 ignored; 0 measured
```

```

` ``
println!("lol");
` ``

asdjnfasd

asd
```

It displays:

```
./build/x86_64-apple-darwin/stage1/bin/rustdoc --test foo.md

running 1 test
test <input> - line 3 ... ok

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

r? @alexcrichton

0 files changed, 0 insertions, 0 deletions