Make QTest::newRow a bit more verbose if a data tag is null.

Currently QTest::newRow function expects to have correct tag string for
each test data. This was enforced by a simple assert. By adding a
an assert with message we might simplify debugging of a test that
generates test cases.

Change-Id: I84ebb397264c99e7bb46d281fd38583dd2d6964c
Reviewed-by: Jason McDonald <jason.mcdonald@nokia.com>

src/testlib/qtestcase.cpp

@@ -2139,6 +2139,7 @@ void QTest::addColumnInternal(int id, const char *name)
 */
 QTestData &QTest::newRow(const char *dataTag)
 {
+    QTEST_ASSERT_X(dataTag, "QTest::newRow()", "Data tag can not be null");
     QTestTable *tbl = QTestTable::currentTestTable();
     QTEST_ASSERT_X(tbl, "QTest::addColumn()", "Cannot add testdata outside of a _data slot.");