tests/test-fuzz-targets.t

py3: replace file() with open() in test-convert-hg-source.t

file() is not present in Python 3.
It also makes sure we read and write in bytes mode on Python 3.

Differential Revision: https://phab.mercurial-scm.org/D2132

#require clang-libfuzzer test-repo
  $ cd $TESTDIR/../contrib/fuzz
  $ make
Just run the fuzzer for five seconds to verify it works at all.
  $ ./bdiff -max_total_time 5