Try to fix the test_aio_suspend crashes in Travis

test_aio_suspend has recently begun to fail on Travis on Linux with
rust-nightly with this error message:

"../sysdeps/pthread/aio_suspend.c:102: do_aio_misc_wait: Assertion
`status == 0 || status == -11' failed"

That error is coming from within glibc itself.  I'm guessing that
glibc's aio_suspend isn't handling signals correctly, probably due to
this bug:
https://sourceware.org/bugzilla/show_bug.cgi?id=13172

test/sys/test_aio.rs

@@ -134,6 +134,12 @@ fn test_fsync_error() {
 #[test]
 #[cfg_attr(all(target_env = "musl", target_arch = "x86_64"), ignore)]
 fn test_aio_suspend() {
+    // Glibc's aio_suspend implementation isn't async-signal-safe, and it causes
+    // assertions (within glibc) on Travis.
+    // https://sourceware.org/bugzilla/show_bug.cgi?id=13172
+    #[cfg(target_env = "glibc")]
+    let _m = ::SIGNAL_MTX.lock().expect("Mutex got poisoned by another test");
+
     const INITIAL: &[u8] = b"abcdef123456";
     const WBUF: &[u8] = b"CDEFG";
     let timeout = TimeSpec::seconds(10);