Note: It is easy to confuse a TestSuite
instance with something that has a static suite method; I know because
I have trouble keeping them straight. Think of something that has
a suite method as simply providing a way to get a meaningful TestSuite instance.
Retuns the rolled up number of tests in this suite; i.e. if the suite
contains other suites, it counts the tests within those suites, not the
suites themselves.