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
722baed173