Allow running just one test with tox

Now, "tox -e py26 -- test.unit.proxy.test_server" runs just that one
test file.

"tox -e py26" still runs all the tests with py26, just like before.

Change-Id: I40db12dd5e7cc8f9388e29b30447f70d3bfc4b28
test 3 date format in functional tests

According to HTTP/1.1, servers MUST accept all three formats:

Sun, 06 Nov 1994 08:49:37 GMT     # RFC 822, updated by RFC 1123
Sunday, 06-Nov-94 08:49:37 GMT    # RFC 850, obsoleted by RFC 1036
Sun Nov 6 08:49:37 1994           # ANSI C's asctime() format

In functional tests, a date value header has 3 kinds of format will be
tested.

Change-Id: I679ed44576208f2a79bffce787cb55bda4b39705
Closes-Bug: #1253207
Merge "test 3 date format in functional tests"
Merge "Allow running just one test with tox"