tests: Pim vrf's need 4.19 or better to run properly not 4.15

tests: Pim vrf's need 4.19 or better to run properly not 4.15

Tests are failing in micronet because linux kernel needs are 4.19
not 4.15

2023-01-11 17:15:06,657.657 INFO: topolog.r1: vtysh command => "show zebra"
2023-01-11 17:15:06,657.657 DEBUG: topolog.r1: LinuxNamespace(r1): cmd_status("['/bin/bash', '-c', 'vtysh  -c "show zebra" 2>/dev/null']", kwargs: {'encoding': 'utf-8', 'stdout': -1, 'stderr': -2, 'shell': False, 'stdin': None})
2023-01-11 17:15:06,729.729 INFO: topolog.r1: vtysh result:
	OS                             Linux(4.15.0-193-generic)

Notice the missing pimreg11 device needed in vrf blue:

2023-01-11 17:15:06,731.731 DEBUG: topolog.r1: LinuxNamespace(r1): cmd_status("['/bin/bash', '-c', 'vtysh  -c "show int brief" 2>/dev/null']", kwargs: {'encoding': 'utf-8', 'stdout': -1, 'stderr': -2, 'shell': False, 'stdin': None})
2023-01-11 17:15:06,781.781 INFO: topolog.r1: vtysh result:
	Interface       Status  VRF             Addresses
	---------       ------  ---             ---------
	blue            up      blue            192.168.0.1/32
	r1-eth0         up      blue            192.168.100.1/24
	r1-eth1         up      blue            192.168.101.1/24

	Interface       Status  VRF             Addresses
	---------       ------  ---             ---------
	erspan0         down    default
	gre0            down    default
	gretap0         down    default
	lo              up      default
	pimreg          up      default

	Interface       Status  VRF             Addresses
	---------       ------  ---             ---------
	r1-eth2         up      red             192.168.100.1/24
	r1-eth3         up      red             192.168.101.1/24
	red             up      red             192.168.0.1/32

While on a 5.4 machine we have this:

mininet310# show int brief
Interface       Status  VRF             Addresses
---------       ------  ---             ---------
blue            up      blue
dummy1          up      blue
dummy2          up      blue
pimreg11        up      blue

As such let's limit the test to a 4.19 kernel or above that our
documentations states we need for proper pim operation.

Signed-off-by: Donald Sharp <sharpd@nvidia.com>