time: enforce current_thread rt for time::pause (#3289)

Pausing time is a capability added to assist with testing Tokio code
dependent on time. Currently, the capability implicitly requires the
current_thread runtime.

This change enforces the requirement by panicking if called from a
multi-threaded runtime.

tokio/tests/time_pause.rs

@@ -0,0 +1,33 @@
+#![warn(rust_2018_idioms)]
+#![cfg(feature = "full")]
+
+use tokio_test::assert_err;
+
+#[tokio::test]
+async fn pause_time_in_main() {
+    tokio::time::pause();
+}
+
+#[tokio::test]
+async fn pause_time_in_task() {
+    let t = tokio::spawn(async {
+        tokio::time::pause();
+    });
+
+    t.await.unwrap();
+}
+
+#[tokio::test(flavor = "multi_thread", worker_threads = 1)]
+#[should_panic]
+async fn pause_time_in_main_threads() {
+    tokio::time::pause();
+}
+
+#[tokio::test(flavor = "multi_thread", worker_threads = 1)]
+async fn pause_time_in_spawn_threads() {
+    let t = tokio::spawn(async {
+        tokio::time::pause();
+    });
+
+    assert_err!(t.await);
+}