Add run-make test for - for -o option

tests/run-make/rustdoc-output-stdout/foo.rs

@@ -0,0 +1 @@
+pub struct Foo;

tests/run-make/rustdoc-output-stdout/rmake.rs

@@ -0,0 +1,25 @@
+// This test verifies that rustdoc `-o -` prints JSON on stdout and doesn't generate
+// a JSON file.
+
+use std::path::PathBuf;
+
+use run_make_support::path_helpers::{cwd, has_extension, read_dir_entries_recursive};
+use run_make_support::rustdoc;
+
+fn main() {
+    // First we check that we generate the JSON in the stdout.
+    rustdoc()
+        .input("foo.rs")
+        .output("-")
+        .arg("-Zunstable-options")
+        .output_format("json")
+        .run()
+        .assert_stdout_contains("{\"");
+
+    // Then we check it didn't generate any JSON file.
+    read_dir_entries_recursive(cwd(), |path| {
+        if path.is_file() && has_extension(path, "json") {
+            panic!("Found a JSON file {path:?}");
+        }
+    });
+}