Fixed many subnet related test cases.

Assertion for `test_create_subnet_3` depends on whether checkbox
or selection list exists.

Fixed many other miscellaneous test cases.
My opinion? Well... I do think that it is best to omit extra keywords from argument lists. For example, let's assume that both of these lines of code are equivalent:

    meth(arg)
    meth(arg=arg)

In the above case, I think the first line of code is preferable, because it doesn't raise unnecessary questions. The use of a keyword argument in a function call makes me think that one of the following is true:

* The function being called accepts `**kwargs`, or
* arguments are being passed in in a non-standard ordering.