tests: open file in binary mode to use POSIX end-of-line style anywhere

Before this patch, "test-context.py" fails on Windows environment,
because "diff" output contains unexpected "\r" character.

Opening the target file in text mode causes this automatic end-of-line
conversion.

tests/test-context.py

@@ -7,7 +7,7 b" repo = hg.repository(u, 'test1', create="
 os.chdir('test1')
 
 # create 'foo' with fixed time stamp
-f = open('foo', 'w')
+f = open('foo', 'wb')
 f.write('foo\n')
 f.close()
 os.utime('foo', (1000, 1000))