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/restart_test.go

@@ -134,7 +134,7 @@ func TestContainerdRestart(t *testing.T) {
 	}
 
 	t.Logf("Pull test images")
-	for _, image := range []string{"busybox", "alpine"} {
+	for _, image := range []string{GetImage(BusyBox), GetImage(Alpine)} {
 		img, err := imageService.PullImage(&runtime.ImageSpec{Image: image}, nil, nil)
 		require.NoError(t, err)
 		defer func() {