Old reference images are not cleaned up

[al3xnag]

If I remove await backstop(assert); from a test or remove/rename a test with await backstop(assert);,
ember test && ember backstop-approve does not remove according (obsolete) image reference from bitmaps_reference/.
This is not a big problem, but since bitmaps_reference/ is committed to git, it would be great to autoremove old image references via ember backstop-approve.

For example:
1.

test('shows specific rental details', async function(assert) {
 await visit('/rentals');
 await click('.grand-old-mansion');
 await backstop(assert);
});

bitmaps_reference/ember-backstoptest_Acceptance__list_rentals__shows_specific_rental_details__assert0_0_document_0_webview.png

- test('shows specific rental details', async function(assert) {
+ test('foobar', async function(assert) {

$ ember test
$ ember backstop-approve

--->

actual:

bitmaps_reference/ember-backstoptest_Acceptance__list_rentals__shows_specific_rental_details__assert0_0_document_0_webview.png
+ bitmaps_reference/ember-backstoptest_Acceptance__list_rentals__foobar__assert0_0_document_0_webview.png

expected:

- bitmaps_reference/ember-backstoptest_Acceptance__list_rentals__shows_specific_rental_details__assert0_0_document_0_webview.png
+ bitmaps_reference/ember-backstoptest_Acceptance__list_rentals__foobar__assert0_0_document_0_webview.png