tests: avoid implicit conversion of str to unicode

On Python 2, str.encode('ascii') implicitly converts the string to unicode and
then back to str.

Since the point of this expression is to convert a bool to a str, the '%r'
conversion specifier can be used instead in the format string.

tests/run-tests.py

@@ -1428,7 +1428,7 b' class Test(unittest.TestCase):'
             )
             hgrc.write(b'[web]\n')
             hgrc.write(b'address = localhost\n')
-            hgrc.write(b'ipv6 = %s\n' % str(self._useipv6).encode('ascii'))
+            hgrc.write(b'ipv6 = %r\n' % self._useipv6)
             hgrc.write(b'server-header = testing stub value\n')
 
             for opt in self._extraconfigopts: