Provide API to fail a test if a warning occurs

This is useful to automatically detect warnings that should not occur, as there is often no way to do so if no state is changed as a result of the problem the warning is alerting us to.

For warnings that should occur, QTest::ignoreMessage() can still be used as previously.

The feature would be especially useful in QML, where it's not possible to fail on warnings.

Here are some potential solutions:

#1 failOnWarnings()/setFailOnWarnings(bool) (https://codereview.qt-project.org/#/c/89178/)

+ Simple API.
+ Can be used (called) from QML.
- No way to limit which warnings cause failures.

void initTestCase()
{
    setFailOnWarnings(true);
}

void testSomething()
{
    setFailOnWarnings(true);
    // ...
}

#2 failOnWarning(const QRegularExpression &) (https://codereview.qt-project.org/c/qt/qtbase/+/375915/1)

+ Simple API.
+ Can limit which warnings cause failures.
+ Can be used (called) from QML.
- No way to stop failing on a certain warning, and as a result;
- No way to implement a simplified "bool failOnWarnings" property for TestCase in QML.

void initTestCase()
{
    failOnWarning(QRegularExpression(".*"));
    // or:
    //failOnWarning(QRegularExpression("Binding loop.*"));
}

void testSomething()
{
    // but can't fail on a warning only in this test:
    //failOnWarning(QRegularExpression("Some warning that shouldn't happen here but is OK elsewhere for some reason"));
}

#3 Context-dependent failOnWarning(const QRegularExpression &)

+ Simple API.
+ Can limit which warnings cause failures.
+ Can be used (called) from QML.
+ Will only ignore the warnings for the scope of the called function, so can be used in init(), initTestCase(), individual test functions, etc.
- No way to stop failing on a certain warning, and as a result;
- No way to implement a simplified "bool failOnWarnings" property for TestCase in QML.
- As behaviour depends on where it's called from, could be slightly more difficult for users to learn.

void initTestCase()
{
    // These will last for the duration of the test case.
    startFailingOnWarning(QRegularExpression(".*"));
    // or: startFailingOnWarning(QRegularExpression("Binding loop.*"));
}

void testSomething()
{
    // These will last for the duration of the test case.
    startFailingOnWarning(QRegularExpression("Some warning that shouldn't happen here but is OK elsewhere for some reason"));
}

#4 addFailOnWarning/startFailingOnWarning + removeFailOnWarning/stopFailingOnWarning

+ Simple API.
+ Can limit which warnings cause failures.
+ Can be used (called) from QML.
+ Can stop failing on a certain warning, and as a result;
+ Can implement a simplified "bool failOnWarnings" property for TestCase in QML.

void initTestCase()
{
    startFailingOnWarning(QRegularExpression(".*"));
    // or: startFailingOnWarning(QRegularExpression("Binding loop.*"));
}

void testSomething()
{
    // This function runs some third party code with a lot of warnings that we can't control...

    const auto warningRegex = QRegularExpression(".*");
    // or: const auto warningRegex = QRegularExpression("Binding loop.*");
    stopFailingOnWarning(warningRegex);
    qScopeGuard([=](){ startFailingOnWarning(warningRegex); });
}