QPROPERTY_TEST_COMPARISON_HELPER: fix extra leading \n in failure output

Once we ported the formatting to use formatFailMessage() in
aa8a4be9e9eae82b6c6b8875371114dbfb3ec06a, it became apparent that the
output contained an extra leading newline compared QCOMPARE use of
formatFailMessage(). We kept the newline in the patch that was picked
to older branches. Since this is a stylistic discrepancy, though,
standardize on no leading newline going forward.

Change-Id: I4badf700900a106d835f3f72e92441d8a8912ff7
Reviewed-by: Ivan Solovev <ivan.solovev@qt.io>
(cherry picked from commit 4bc81fe5d1d1e4587075a82a45be9b34825b5233)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

src/testlib/qtestresult.cpp

@@ -372,7 +372,7 @@ QTest::Internal::formatPropertyTestHelperFailure(char *msg, size_t maxMsgLen,
                                                  const char *actual, const char *expected,
                                                  const char *actualExpr, const char *expectedExpr)
 {
-    formatFailMessage(msg, maxMsgLen, "\nComparison failed!", // ### why leading \n?
+    formatFailMessage(msg, maxMsgLen, "Comparison failed!",
                       actual, expected, actualExpr, expectedExpr,
                       QTest::ComparisonOperation::CustomCompare);
     return msg;