QTestlib/selftests: Fix failures due to logging system output

Silence debug output by setting QT_LOGGING_RULES
to turn off all debug output.

Task-number: QTQAINFRA-1631
Change-Id: I5c2366b4fe4bac341dcfd92f68b6da8071c5b089
Reviewed-by: Mitch Curtis <mitch.curtis@qt.io>

tests/auto/testlib/selftests/tst_selftests.cpp

@@ -404,6 +404,7 @@ tst_Selftests::tst_Selftests()
 void tst_Selftests::initTestCase()
 {
     QVERIFY2(tempDir.isValid(), qPrintable(tempDir.errorString()));
+    qputenv("QT_LOGGING_RULES", QByteArrayLiteral("*.debug=false")); // Silence any debug output
     //Detect the location of the sub programs
     QString subProgram = QLatin1String("float/float");
 #if defined(Q_OS_WIN)