Rollup merge of rust-lang#124681 - risc0:erik/fix-test, r=joboet

zkvm: fix run_tests

`zkvm` is single-threaded, similar to `emscripten` and `wasm`. The `cfg` for `zkvm` seems to have been dropped. This PR adds the `cfg` again.

library/test/src/lib.rs

@@ -589,7 +589,9 @@ pub fn run_test(
             // If the platform is single-threaded we're just going to run
             // the test synchronously, regardless of the concurrency
             // level.
-            let supports_threads = !cfg!(target_os = "emscripten") && !cfg!(target_family = "wasm");
+            let supports_threads = !cfg!(target_os = "emscripten")
+                && !cfg!(target_family = "wasm")
+                && !cfg!(target_os = "zkvm");
             if supports_threads {
                 let cfg = thread::Builder::new().name(name.as_slice().to_owned());
                 let mut runtest = Arc::new(Mutex::new(Some(runtest)));