Reorder mux test to account for async reads

The behaviour of the `'readable'` event changed, or was tightened up,
in

    nodejs/node#17979

such that it is _always_ called on the next tick. This change appears
first in Node.JS v10.0).

Since we rely on `'readable' in the multiplexing, it means that we
have to be more careful about when we wait for the next event loop to
come around in tests.

The tests tend to be much more brittle than live code with respect to
the order things happen, since they attempt to nail down precise
states or orderings (e.g., "the `unpipe` happened exactly between
these writes").

test/mux.js

@@ -37,7 +37,7 @@ test("single input", function(done) {
   var mux = new Mux(output);
   mux.pipeFrom(input);
 
-  var data = [1,2,3,4,5,6,7,8,9]; 
+  var data = [1,2,3,4,5,6,7,8,9];
   // not 0, it's treated specially by PassThrough for some reason. By
   // 'specially' I mean it breaks the stream. See e.g.,
   // https://github.com/isaacs/readable-stream/pull/55
@@ -112,16 +112,20 @@ test("unpipe", function(done) {
 
   schedule(function() {
     pipedData.forEach(input.write.bind(input));
-    mux.unpipeFrom(input);
 
     schedule(function() {
-      unpipedData.forEach(input.write.bind(input));
-      input.end();
+      mux.unpipeFrom(input);
+
       schedule(function() {
-        // exhaust so that 'end' fires
-        var v; while (v = input.read());
+        unpipedData.forEach(input.write.bind(input));
+        input.end();
+        schedule(function() {
+          // exhaust so that 'end' fires
+          var v; while (v = input.read());
+        });
       });
     });
+
   });
 
   input.on('end', function() {