We also track the instance on which a given method is invoked and verify
that the `ExtensionContext` contains consistent information. This
prepares for future changes to `Lifecycle.PER_EXECUTION`.

Previously, `dumpReproducer` was called for `@FuzzTest`s using Autofuzz,
which is implemented as a static fuzz target method.

Each execution uses its own dedicated test class instance and also runs
preprocessors.

See the comment in `JUnitLifecycleMethodsInvoker#beforeFirstExecution`
for an explanation of how this still falls short of emulating default
JUnit behavior.

The check generates a warning for essentially all our tests and doesn't
seem to be easy to make more precise.

Previously, IDE executions of JUnit fuzz tests registered a
`findingHandler` in `FuzzTargetRunner` whereas CLI executions did not.
This lead to inconsistent behavior that was hard to reason about and a
lack of feature parity between the two modes (e.g. `--keep_going` was
only supported on the CLI). 

Instead, we now use a `findingHandler` to report the last, "fatal",
finding in structured form to `FuzzTestExecutor`, with all other
findings having their stack traces printed. `JUnitRunner` now handles
findings from lifecycle methods correctly, including for the exit code.