signal: Refactor Signal tests

* Added timeouts to all tests that were missing them
 - any issue we have will likely result in deadlocks/starvation so its
   best if all tests quickly timeout rather than require getting killed
   or have the CI timeout itself
* Added a `support` module and put a bunch of helpers there to DRY the
tests

tests/drop_multi_loop.rs

@@ -0,0 +1,39 @@
+#![cfg(unix)]
+
+extern crate libc;
+
+pub mod support;
+use support::*;
+
+const TEST_SIGNAL: libc::c_int = libc::SIGUSR1;
+
+#[test]
+fn dropping_loops_does_not_cause_starvation() {
+    let (mut rt, signal) = {
+        let mut first_rt = CurrentThreadRuntime::new()
+            .expect("failed to init first runtime");
+
+        let first_signal = run_with_timeout(&mut first_rt, Signal::new(TEST_SIGNAL))
+            .expect("failed to register first signal");
+
+        let mut second_rt = CurrentThreadRuntime::new()
+            .expect("failed to init second runtime");
+
+        let second_signal = run_with_timeout(&mut second_rt, Signal::new(TEST_SIGNAL))
+            .expect("failed to register second signal");
+
+        drop(first_rt);
+        drop(first_signal);
+
+        (second_rt, second_signal)
+    };
+
+    send_signal(TEST_SIGNAL);
+
+    let signal_future = signal.into_future()
+        .map_err(|(e, _)| e);
+
+    run_with_timeout(&mut rt, signal_future)
+        .expect("failed to get signal");
+}
+