Remove Q_ASSERT from QLibrary autotest

Report a regular test failure rather than aborting when an unknown
operation is requested.

Change-Id: I3678f42f9e9f895d8d0d49a768d4c8b575d1863f
Task-number: QTBUG-17582
Reviewed-by: Rohan McGovern
(cherry picked from commit 22aa68ecec96da4d14a4c9fe2f6e4a337b13b4a3)

tests/auto/qlibrary/tst_qlibrary.cpp

@@ -468,7 +468,7 @@ void tst_QLibrary::errorString()
             }
             break;}
         default:
-            Q_ASSERT(0);
+            QFAIL(qPrintable(QString("Unknown operation: %1").arg(operation)));
             break;
     }
     QRegExp re(errorString);