Support for the number of test cases running inside IEventListener

**Description**
I'm writing a progress bar for the tests and it would be nice to have the total number of test cases to be executed inside the IEventListener constructor or IEventListener::testRunStarting method.

**Additional context**
I was able to achieve this by copy/pasting code from the `TestGroup` constructor to the `IEventListener` implementation but this is not a nice solution.
```
auto tests = std::set{};
auto const& allTestCases = getAllTestCasesSorted(*m_config);
auto const& testSpec = m_config->testSpec();
if (!testSpec.hasFilters())
{
for (auto const& test : allTestCases)
{
if (!test.getTestCaseInfo().isHidden())
{
tests.emplace(&test);
}
}
}
else
{
auto matches = testSpec.matchesByFilter(allTestCases, *m_config);
for (auto const& match : matches)
{
tests.insert(match.tests.begin(), match.tests.end());
}
}
m_testCount = tests.size();
```