tests: Refactors container image usage

Currently, the cri-integration tests do not work on Windows due to various reasons.
One of the reasons is because all the tests are using Linux-specific images. This
commit refactors the image pulling / usage in the cri-integration tests, making it
easier to update, and easier to configure the a custom registry to pull those images
from.

For Windows runs, custom registries can be created, which will also contain Windows
images, and the cri-integration tests can be configured to use those registries by
specifying the "--image-list" argument, a TOML file which will contain an alternative
mapping of the default images.

Signed-off-by: Claudiu Belu <cbelu@cloudbasesolutions.com>

integration/pod_dualstack_test.go

@@ -46,8 +46,8 @@ func TestPodDualStack(t *testing.T) {
 		assert.NoError(t, runtimeService.RemovePodSandbox(sb))
 	}()
 
-	const (
-		testImage     = "busybox"
+	var (
+		testImage     = GetImage(BusyBox)
 		containerName = "test-container"
 	)
 	t.Logf("Pull test image %q", testImage)