Set encoding in test driver

Python3 handles encoding differently, and files are loaded
according to the locale set on the system.

This means that when building on a locale-less chroot,
the tests will always fail because it will try to load utf-8
and encode it as ASCII.

This forces the encoding. The tests are encoded in utf-8 anyway
so any other encoding set by the locale would cause a failure.

driver.py

@@ -40,7 +40,7 @@ tests_path = 'test/'
 
 def readfile(fname):
     '''Reads a file as string and returns its content'''
-    fd = open(fname)
+    fd = open(fname,encoding='utf-8')
     expr = fd.read()
     fd.close()
     return expr