Fixes:https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=69350**`IsADirectoryError`**Fuzzer provided input data can sometimes produce filenames that looklike directories and raise `IsADirectoryError` exceptions which crashthe fuzzer. This commit catches those cases and returns -1 to instructlibfuzzer that the inputs are not valuable to add to the corpus.**`FileExistsError`**Similar to the above, this is a possible exception case produced by thefuzzed data and not a bug so its handled the same.

Adds a utility function to limit the maximum file name legnth producedby the fuzzer to a max size dictated by the host its run on.

The fuzzer was having trouble analyzing `fuzz_submodule.py` when usingthe `atheris.instrument_imports()` context manager. Switching to`atheris.instrument_all()` instead slightly increases the startup timefor the fuzzer, but significantly improves the fuzzing engines abilityto identify new coverage.The changes here also disable warnings that are logged to `stdout` fromthe SUT. These warnings are expected to happen with some inputs andclutter the fuzzer output logs. They can be optionally re-enabled fordebugging by passing a flag o the Python interpreter command line orsetting the `PYTHONWARNINGS` environment variable.

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