selftests: fib_tests: Fix 'Command line is not complete' errors

A couple of tests are verifying a route has been removed. The helper
expects the prefix as the first part of the expected output. When
checking that a route has been deleted the prefix is empty leading
to an invalid ip command:

  $ ip ro ls match
  Command line is not complete. Try option "help"

Fix by moving the comparison of expected output and output to a new
function that is used by both check_route and check_route6. Use the
new helper for the 2 checks on route removal.

Also, remove the reset of 'set -x' in route_setup which overrides the
user managed setting.

Fixes: d69faad7

 ("selftests: fib_tests: Add prefix route tests with metric")
Signed-off-by: David Ahern <dsahern@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>