Migrate missing_optional_backends_are_not_fatal to BookTest

tests/alternative_backends.rs

@@ -6,12 +6,6 @@ use std::fs;
 use std::path::Path;
 use tempfile::{Builder as TempFileBuilder, TempDir};
 
-#[test]
-fn missing_optional_backends_are_not_fatal() {
-    let (md, _temp) = dummy_book_with_backend("missing", "trduyvbhijnorgevfuhn", true);
-    assert!(md.build().is_ok());
-}
-
 #[test]
 fn alternate_backend_with_arguments() {
     let (md, _temp) = dummy_book_with_backend("arguments", "echo Hello World!", false);

tests/testsuite/renderer.rs

@@ -91,3 +91,17 @@ fn missing_renderer() {
 "#]]);
     });
 }
+
+// Optional missing is not an error.
+#[test]
+fn missing_optional_not_fatal() {
+    BookTest::from_dir("renderer/missing_optional_not_fatal").run("build", |cmd| {
+        cmd.expect_stdout(str![[""]]).expect_stderr(str![[r#"
+[TIMESTAMP] [INFO] (mdbook::book): Book building has started
+[TIMESTAMP] [INFO] (mdbook::book): Running the missing backend
+[TIMESTAMP] [INFO] (mdbook::renderer): Invoking the "missing" renderer
+[TIMESTAMP] [WARN] (mdbook::renderer): The command `trduyvbhijnorgevfuhn` for backend `missing` was not found, but was marked as optional.
+
+"#]]);
+    });
+}

tests/testsuite/renderer/missing_optional_not_fatal/book.toml

@@ -0,0 +1,4 @@
+[output.missing]
+command = "trduyvbhijnorgevfuhn"
+optional = true
+