QCOMPARE/QVERIFY: fix huge pessimisation in QTestResult::reportResult

The old code allocated a stack buffer, but asked the runtime to
zero-initialize it. That's 1KiB of writes to the stack on every
QCOMPARE and QVERIFY before any actual work is done. Fixing this
little laissez-faire to just initialize the first character in the
buffer results in nice little speedups of ~40%.

This fixes the issue in reportResult(), a Cut'n'paste from compare()
and verify(), which have since been fixed by a previous commit.

Change-Id: I5cad57299490925b88e768dc751304699274db2e
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@qt.io>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
Reviewed-by: Mårten Nordheim <marten.nordheim@qt.io>

src/testlib/qtestresult.cpp

@@ -629,7 +629,8 @@ bool QTestResult::reportResult(bool success, qxp::function_ref<const char *()> l
                                const char *failureMessage)
 {
     const size_t maxMsgLen = 1024;
-    char msg[maxMsgLen] = {'\0'};
+    char msg[maxMsgLen];
+    msg[0] = '\0';
 
     QTEST_ASSERT(lhsExpr);
     QTEST_ASSERT(rhsExpr);