Fix tests and enable running tests in CI

[antoninbas]

There should be a convenient way to run tests for this repo. Right now, you pretty much need an instance of the OVSDB server listening on the correct TCP port. Tests sometimes require root permissions and use sudo liberally.

The following should be done:

• fix tests (right now they fail even when all the preconditions are met - see below)
• make test should just work
• CI (Github workflows) should be able to run the tests; to keep this repo simple we want to run all the tests in Github workflows (no Jenkins job)
• separate tests between unit tests (no dependency on ovsdb-server / ovs-vswitch, don't need to run as root) and integrations tests; integration tests should be run inside a Docker container

--- FAIL: TestPacketIn_PacketOut_IPv6 (3.01s)
panic: interface conversion: util.Message is *protocol.IPv6, not *util.Buffer [recovered]
	panic: interface conversion: util.Message is *protocol.IPv6, not *util.Buffer

goroutine 904 [running]:
testing.tRunner.func1.2(0x69f480, 0xc0005ea240)
	/usr/local/go/src/testing/testing.go:1143 +0x332
testing.tRunner.func1(0xc000617080)
	/usr/local/go/src/testing/testing.go:1146 +0x4b6
panic(0x69f480, 0xc0005ea240)
	/usr/local/go/src/runtime/panic.go:965 +0x1b9
antrea.io/ofnet/ofctrl.testPacketInOut(0xc000617080, 0xc0005e4a20, 0x6f172d, 0x8, 0x68e201)
	/home/abas/ofnet/ofctrl/ofpacket_test.go:232 +0x1c9f
antrea.io/ofnet/ofctrl.TestPacketIn_PacketOut_IPv6(0xc000617080)
	/home/abas/ofnet/ofctrl/ofpacket_test.go:72 +0x205
testing.tRunner(0xc000617080, 0x70af48)
	/usr/local/go/src/testing/testing.go:1193 +0xef
created by testing.(*T).Run
	/usr/local/go/src/testing/testing.go:1238 +0x2b3
FAIL	antrea.io/ofnet/ofctrl	51.621s