Introduce VPP integration tests

[ondrej-fabry]

This PR adds another GitHub Workflow test.yaml for executing tests against a running VPP instance. The tests run against 3 latest releases of VPP, 22.10, 22.06, 22.02, installed from the official debian package published by FD.io to packagecloud.

All the testing against a running VPP instance are located under test/integration. The test/vpptesting package provides a dead simple way to setup fresh VPP instance and is used in all tests. The tests are written as traditional Go unit test. There are few basic test cases included in this PR:

• TestExamples - runs all the GoVPP examples
• TestVersion - checks if the version retrieved via binary API is non-empty:
  

  govpp/test/integration/binapi_test.go

  Lines 25 to 39 in eb3dddd

  	func TestVersion(t *testing.T) {
  	test := vpptesting.SetupVPP(t)
  	vpeRPC := vpe.NewServiceClient(test.Conn)
  	versionInfo, err := vpeRPC.ShowVersion(context.Background(), &vpe.ShowVersion{})
  	if err != nil {
  	t.Fatalf("getting version failed: %v", err)
  	}
  	t.Logf("VPP version: %v", versionInfo.Version)
  	if versionInfo.Version == "" {
  	t.Fatal("expected VPP version to not be empty")
  	}
  	}

• TestStatClient* - runs all stats client methods

You can run the integration tests by running:

# Run against latest released VPP version
make test-integration

# Run against VPP 22.10 (stable/2210 branch)
make test-integration VPP_REPO=2210

# Run against VPP development version (master branch)
make test-integration VPP_REPO=master