Fully match stderr, return if not nightly, shorten file

tests/testsuite/doc.rs

@@ -1171,7 +1171,10 @@ fn doc_workspace_open_help_message() {
 #[cargo_test]
 #[cfg(not(windows))] // `echo` may not be available
 fn doc_extern_map_local() {
-    assert!(is_nightly());
+    if !is_nightly() {
+        // -Zextern-html-root-url is unstable
+        return;
+    }
 
     let p = project()
         .file(
@@ -1183,21 +1186,20 @@ fn doc_extern_map_local() {
             "#,
         )
         .file("src/lib.rs", "")
+        .file(".cargo/config.toml", "doc.extern-map.std = 'local'")
         .build();
 
-    p.change_file(
-        ".cargo/config.toml",
-        r#"
-[doc.extern-map]
-std = "local"
-    "#,
-    );
-
     p.cargo("doc -v --no-deps -Zrustdoc-map --open")
         .env("BROWSER", "echo")
         .masquerade_as_nightly_cargo()
-        .with_stderr_contains("[..] Documenting foo v0.1.0 ([..])")
-        .with_stderr_does_not_contain("warning: unused config key `doc.extern-map` [..]")
+        .with_stderr(
+            "\
+[DOCUMENTING] foo v0.1.0 [..]
+[RUNNING] `rustdoc --crate-type lib --crate-name foo src/lib.rs [..]--crate-version 0.1.0`
+[FINISHED] [..]
+     Opening [CWD]/target/doc/foo/index.html
+",
+        )
         .run();
 }