tests: write out file using bytes I/O
The encoding of sys.stdout varies between Python versions. So
using a one-liner to write a file from a Unicode string is not
deterministic.
This commit writes out the file using bytes I/O to ensure we
have exactly the bytes we want in the file.
This change fixes a test failure in Python 3.5/3.6.
Differential Revision: https://phab.mercurial-scm.org/D7226
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