Quicker termination for eth --test

Before this commit, `eth --test` took half seconds to terminate after a SIGINT.

Since retesteth required this after every test, most of the testing time
was spent during this half seconds.  ping @winsvega

Author: pirapira

libethereum/Client.cpp

@@ -695,7 +695,7 @@ void Client::doWork(bool _doWait)
     if (!m_syncBlockQueue && !m_syncTransactionQueue && (_doWait || isSealed))
     {
         std::unique_lock<std::mutex> l(x_signalled);
-        m_signalled.wait_for(l, chrono::seconds(1));
+        m_signalled.wait_for(l, chrono::milliseconds(30));
     }
 }