Compiletest path normalization fails in windows if path is inside a string

[celinval]

In the initial version of #132161, a Windows CI job because the stdout contained a string containing the test path that wasn't normalized.

I'm pasting the error as described in #132161 (comment):

---- [ui] tests\ui\stable-mir-print\operands.rs stdout ----
$DIR\operands.rs
$DIR\operands.rs
\a\rust\rust\tests\ui\stable-mir-print\operands.rs
$DIR\operands.rs
Saved the actual stdout to "C:\\a\\rust\\rust\\build\\x86_64-pc-windows-msvc\\test\\ui\\stable-mir-print\\operands\\operands.stdout"


226     debug x => _1;
227     debug z => _2;
228     bb0: {
-         _0 = {closure@Span { id: 105, repr: "$DIR/operands.rs:44:5: 44:19" }}(_1, _2);
+         _0 = {closure@Span { id: 105, repr: "C:/a/rust/rust/tests/ui/stable-mir-print/operands.rs:44:5: 44:19" }}(_1, _2);
231     }
232 }