Update time test expectation to account for runtime variations (#31)

Fixes https://github.com/dart-lang/pool/issues/30
diff --git a/test/pool_test.dart b/test/pool_test.dart
index d4d7789..77ef13d 100644
--- a/test/pool_test.dart
+++ b/test/pool_test.dart
@@ -553,7 +553,7 @@
               expect(finishedItems, itemCount);
 
               final expectedElapsed =
-                  delayedToStringDuration.inMicroseconds * 3;
+                  delayedToStringDuration.inMicroseconds * 4;
 
               expect((watch.elapsed ~/ itemCount).inMicroseconds,
                   lessThan(expectedElapsed / (poolSize - otherTaskCount)),