Fix TestIPv4FibCrud interdependencies.

Fix TestIPv4FibCrud interdependencies.

The tests fail if run independently or out of order.
This change breaks the dependency between the tests.

ERROR: Add 1k routes
------------------------------------------------------------------------------
Traceback (most recent call last):
  File "/vpp/test/test_ip4.py", line 514, in test_3_add_new_routes
    self.deleted_routes.remove(x)
ValueError: list.remove(x): x not in list

Change-Id: I344ceba6bd8b86556f92e50080be6c43092b9faf
Signed-off-by: Paul Vinciguerra <pvinci@vinciconsulting.com>