rt: change max_threads to max_blocking_threads (#3287)

Fixes #2802

tokio/tests/rt_threaded.rs

@@ -331,7 +331,7 @@ fn coop_and_block_in_place() {
         // runtime worker yielded as part of `block_in_place` and guarantees the
         // same thread will reclaim the worker at the end of the
         // `block_in_place` call.
-        .max_threads(1)
+        .max_blocking_threads(1)
         .build()
         .unwrap();
 
@@ -375,9 +375,18 @@ fn coop_and_block_in_place() {
 
 // Testing this does not panic
 #[test]
-fn max_threads() {
+fn max_blocking_threads() {
     let _rt = tokio::runtime::Builder::new_multi_thread()
-        .max_threads(1)
+        .max_blocking_threads(1)
+        .build()
+        .unwrap();
+}
+
+#[test]
+#[should_panic]
+fn max_blocking_threads_set_to_zero() {
+    let _rt = tokio::runtime::Builder::new_multi_thread()
+        .max_blocking_threads(0)
         .build()
         .unwrap();
 }