rewrite rustdoc-default-output to rmake

src/tools/tidy/src/allowed_run_make_makefiles.txt

@@ -19,7 +19,6 @@ run-make/issue-36710/Makefile
 run-make/issue-47551/Makefile
 run-make/issue-69368/Makefile
 run-make/issue-84395-lto-embed-bitcode/Makefile
-run-make/issue-88756-default-output/Makefile
 run-make/jobserver-error/Makefile
 run-make/libs-through-symlinks/Makefile
 run-make/libtest-json/Makefile

tests/run-make/issue-88756-default-output/Makefile

@@ -1,4 +0,0 @@
-include ../tools.mk
-
-all:
-	$(BARE_RUSTDOC) 2>&1 | sed -E 's@/nightly/|/beta/|/stable/|/1\.[0-9]+\.[0-9]+/@/$$CHANNEL/@g' | diff - output-default.stdout

tests/run-make/issue-88756-default-output/README.md

@@ -1 +0,0 @@
-This is a test to verify that the default behavior of `rustdoc` is printing out help output instead of erroring out (#88756).

tests/run-make/issue-88756-default-output/x.rs

@@ -1 +0,0 @@
-// nothing to see here

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

@@ -0,0 +1,16 @@
+// Calling rustdoc with no arguments, which should bring up a help menu, used to
+// cause an error as rustdoc expects an input file. Fixed in #98331, this test
+// ensures the output of rustdoc's help menu is as expected.
+// See https://github.com/rust-lang/rust/issues/88756
+
+use run_make_support::{bare_rustdoc, diff};
+
+fn main() {
+    let out = bare_rustdoc().run().stdout_utf8();
+    diff()
+        .expected_file("output-default.stdout")
+        .actual_text("actual", out)
+        // replace the channel type in the URL with $CHANNEL
+        .normalize(r"nightly/|beta/|stable/|1\.[0-9]+\.[0-9]+/", "$$CHANNEL/")
+        .run();
+}