Separate positive and negative test generation and refactor

 * Positive and negative tests are now produced entirely in isolation,
if 'both' test types are chosen all positive tests are generated,
then all the negative tests are generated, without combining the two
 * Moved tensor generation out of serialize test and into it's own function

Signed-off-by: Matthew Haddon <matthew.haddon@arm.com>
Change-Id: Id8e9023d2c2966a0b14ae83d2d848a66cb125fcb