Rollup merge of #35574 - badboy:emscripten-test-fixes, r=brson

Emscripten test fixes

This picks up parts of #31623 to disable certain tests that emscripten can't run, as threads/processes are not supported.
I re-applied @tomaka's changes manually, I can rebase those commits with his credentials if he wants.

It also disables jemalloc for emscripten (at least in Rustbuild, I have to check if there is another setting for the same thing in the old makefile approach).

This should not impact anything for normal builds.

src/bootstrap/sanity.rs

@@ -98,7 +98,8 @@ pub fn check(build: &mut Build) {
         if target.contains("rumprun") ||
            target.contains("bitrig") ||
            target.contains("openbsd") ||
-           target.contains("msvc") {
+           target.contains("msvc") ||
+           target.contains("emscripten") {
             build.config.use_jemalloc = false;
         }