test: enable fuzz regression tests by default

This ensures that the fuzz test code is also built by default.
It also increases the test coverage a bit. Compiling the tests
*with* sanitizers is painfully slow, so this is not enabled. But
just compiling them sauté is hardly noticable. Running the tests
increases the test count and runtime:
  622 tests, 26 s
to
  922 tests, 35 s
I think this is acceptable.