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

tests/test-highlight.t

@@ -966,7 +966,9 b' errors encountered'
   $ cd ..
   $ hg init eucjp
   $ cd eucjp
-  $ "$PYTHON" -c 'print("\265\376")' >> eucjp.txt  # Japanese kanji "Kyo"
+  >>> with open('eucjp.txt', 'wb') as fh:
+  ...     # Japanese kanji "Kyo"
+  ...     fh.write(u'\265\376'.encode('utf-8')) and None
   $ hg ci -Ama
   adding eucjp.txt
   $ hgserveget () {