[FIX] web: Explicitly decode the JS files in tests.

When in Python 3, then function `open` defaults to open the file in text mode
with and the encoding (if not passed) is taken from
`locale.getpreferredencoding`.

Using `getpreferredencoding` is not necessarily a good practice.  Because
files might no be encoded that way.

When testing Odoo not in runbot (which has LANG set to C.UTF-8) you may get:

Traceback (most recent call last):
  File "/home/gitlab-runner/src/odoo11/addons/web/tests/test_js.py", line 24, in test_check_suite
    self._check_only_call('web.qunit_suite')
  File "/home/gitlab-runner/src/odoo11/addons/web/tests/test_js.py", line 37, in _check_only_call
    if RE_ONLY.search(fp.read()):
  File "/home/gitlab-runner/virtualenvs/odoo11-MlnlRiTaP2/lib/python3.5/encodings/ascii.py", line 26, in decode
    return codecs.ascii_decode(input, self.errors)[0]
UnicodeDecodeError: 'ascii' codec can't decode byte 0xc3 in position 4757: ordinal not in range(128)

The proposed solution is to read the file in binary mode, and decode to
unicode/str ourselves using utf-8.  We don't use the encoding argument of
`open`, so that the code works in both Python 3 and Python 2.

PR  #24434

addons/web/tests/test_js.py

@@ -33,6 +33,6 @@ class WebSuite(odoo.tests.HttpCase):
             filename = asset['filename']
             if not filename or asset['atype'] != 'text/javascript':
                 continue
-            with open(filename, 'r') as fp:
-                if RE_ONLY.search(fp.read()):
+            with open(filename, 'rb') as fp:
+                if RE_ONLY.search(fp.read().decode('utf-8')):
                     self.fail("`QUnit.only()` used in file %r" % asset['url'])