Mark tests ignored on emscripten rather than omitting

This way they still appear in `cargo test`'s output.

test_suite/tests/compiletest.rs

@@ -1,4 +1,4 @@
-#[cfg(not(target_os = "emscripten"))]
+#[cfg_attr(target_os = "emscripten", ignore)]
 #[rustversion::attr(not(nightly), ignore)]
 #[test]
 fn ui() {

test_suite/tests/expandtest.rs

@@ -1,6 +1,6 @@
-#[cfg(not(target_os = "emscripten"))]
-#[rustversion::attr(not(nightly), ignore)]
+#[cfg_attr(target_os = "emscripten", ignore)]
 #[cfg_attr(not(cargo_expand), ignore)]
+#[rustversion::attr(not(nightly), ignore)]
 #[test]
 fn expandtest() {
     macrotest::expand("tests/expand/*.rs");