selftests: tc-testing: fix list_categories() crash on list type

selftests: tc-testing: fix list_categories() crash on list type

list_categories() builds a set directly from the 'category'
field of each test case. Since 'category' is a list,
set(map(...)) attempts to insert lists into a set, which
raises:

  TypeError: unhashable type: 'list'

Flatten category lists and collect unique category names
using set.update() instead.

Signed-off-by: Naveen Anandhan <mr.navi8680@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>