No longer try to verify kaniko dir isn't snapshotted

This test had previously (before #231) been making a change to a file in
the kaniko dir, then checking that it isn't being snapshotted. This was
to test the whitelisting logic, which makes sure that changes to /kaniko
aren't included in images. However the test creates a temporary dir, so
the kaniko dir is actually in /tmp/<some temp dir>/kaniko, and
in #231 the logic was simplified to no longer have a special case for
tests. The test continued to pass because `MaybeAdd` noticed that the
kaniko file wasn't changing, and didn't add it. After changing this to
always add the files, it revealed that this was left behind by accident.

I also opened #307 to add integration test coverage for this logic.

I also marked `CheckErrorAndDeepEqual` as a helper function so that when
it fails, the line number reported is where that was called.

pkg/snapshot/snapshot_test.go

@@ -141,7 +141,6 @@ func TestSnapshotFiles(t *testing.T) {
 	}
 	filesToSnapshot := []string{
 		filepath.Join(testDir, "foo"),
-		filepath.Join(testDir, "kaniko/file"),
 	}
 	contents, err := snapshotter.TakeSnapshot(filesToSnapshot)
 	if err != nil {

testutil/util.go

@@ -40,6 +40,7 @@ func SetupFiles(path string, files map[string]string) error {
 }
 
 func CheckErrorAndDeepEqual(t *testing.T, shouldErr bool, err error, expected, actual interface{}) {
+	t.Helper()
 	if err := checkErr(shouldErr, err); err != nil {
 		t.Error(err)
 		return