Reuse `TestClass` for parameterized tests.

The `BlockJUnit4ClassRunnerWithParameters` created a new instance of
`TestClass` for each parameter set. This led to repeated class
scanning and noticeable memory allocation. Reusing the `TestClass`
avoids theses side effects.

Fixes #1046.
Reuse TestClass.

Since bb34f67 the BlockJUnit4ClassRunner can be created with a
TestClass. This avoids recreating the TestClass.