tests: net: websocket: Fix test execution on 64-bit platforms

Instead of passing the test parameters to the websocket function, by
casting the pointer to integer (which may not work on 64-bit platform
due to int/pointer size mismatch), let the test allocate a file
descriptor, and initialize it with test context. The tested functions
expect a file descriptor as an argument anyway, so it's a more intuitive
approach.

The conditional test code within WS implementation can retrieve the test
context by using FD APIs to obtain the object represented by the FD.

Signed-off-by: Robert Lubos <robert.lubos@nordicsemi.no>