test: net: buf_simple: Add unit tests for simple buffers

This adds set of unit tests for simple network buffers.
The suite is based of the test cases that are already defined for
network buffers.
Those test_net_buf_byte_order test case have been split to
smaller tests testing one functionality at a time.

Signed-off-by: Mariusz Skamra <mariusz.skamra@codecoup.pl>