Hash saved test filenames from their input bytes

[tdcinaz]

Fixes issue #309

Summary

• replace the random saved-test filename generator with an MD5 of the bytes written to disk
• keep saved testcase stems aligned with the existing Python symex path and README examples
• add an integration test that checks saved testcase filenames match the MD5 of their contents

Testing

• python -m py_compile tests/test_saved_case_hashes.py
• clang -D__unix=1 -Isrc/include -Isrc/lib -I. -c src/lib/DeepState.c -o /tmp/deepstate-saved-case-hash.o

The full integration test was added but not executed here because this clean PR branch was intentionally kept free of the local macOS build fixes needed to build DeepState on this machine.

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