Don't build test helpers for wasm32

Author: Zoxc

src/bootstrap/test.rs

@@ -1016,7 +1016,10 @@ impl Step for Compiletest {
         // Also provide `rust_test_helpers` for the host.
         builder.ensure(native::TestHelpers { target: compiler.host });
 
-        builder.ensure(native::TestHelpers { target });
+        // wasm32 can't build the test helpers
+        if !target.contains("wasm32") {
+            builder.ensure(native::TestHelpers { target });
+        }
         builder.ensure(RemoteCopyLibs { compiler, target });
 
         let mut cmd = builder.tool_cmd(Tool::Compiletest);