Reuse `TestClass` for parameterized tests.

The `BlockJUnit4ClassRunnerWithParameters` created a new instance of
`TestClass` for each parameter set. This was slowing down parameterized
tests with plenty of parameter sets. Reusing the `TestClass` speeds up
such tests.
Reuse `TestClass`.

Since 1e7cc15 the BlockJUnit4ClassRunner can be created with a
`TestClass`. This avoids recreating the `TestClass`.