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/image_load_test.go

@@ -32,8 +32,8 @@ import (
 
 // Test to load an image from tarball.
 func TestImageLoad(t *testing.T) {
-	testImage := "busybox:latest"
-	loadedImage := "docker.io/library/" + testImage
+	testImage := GetImage(BusyBox)
+	loadedImage := testImage
 	_, err := exec.LookPath("docker")
 	if err != nil {
 		t.Skipf("Docker is not available: %v", err)