net: lib http: Added tests for Content-Range to http parser.

Three tests has been added for Content-Range functionality in http
parser:
- test_content_range_supplied: Checks if parser handles range correctly.
- test_content_range_asterisk_total: Checks if parser interprets
astersk as no total size supplied.
- test_double_content_range_error: Checks if parser rejects header with
repeated Content-Range field.

Signed-off-by: Piotr Radecki <piotr.radecki@jrdltd.co.uk>