flask: fix fuzz json

[rdesfo]

Resolves the following message:

=== Uncaught Python exception: ===
NameError: name 'os' is not defined
Traceback (most recent call last):
  File "/opt/fuzz_json.py", line 33, in TestOneInput
    app = FuzzFlask("flask_test", root_path=os.path.dirname(__file__))
NameError: name 'os' is not defined

==3849== ERROR: libFuzzer: fuzz target exited
SUMMARY: libFuzzer: fuzz target exited
MS: 0 ; base unit: 0000000000000000000000000000000000000000

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[github-actions[bot]]

rdesfo is a new contributor to projects/flask. The PR must be approved by known contributors before it can be merged. The past contributors are: illia-v, ret2libc, manunio, arthurscchan