apiv2 tests: use quay.io/libpod/testimage:20210610 for platform tests
The quay.io/libpod/testimage:20210610 is known not to change and to
remain stable over time.  While the same should apply for alpine image
on quay.io/libpod, we've seen it flake and return the wrong image.
The reasons for that observation are unknown.
Fixes:
@@ -53,8 +53,8 @@ t POST "images/create?fromImage=alpine" 200 .error~null .status~".*Download comp
t POST "images/create?fromImage=alpine&tag=latest" 200
# 10977 - handle platform parameter correctly
-t POST "images/create?fromImage=alpine&platform=linux/arm64" 200
-t GET "images/alpine/json" 200 \
+t POST "images/create?fromImage=testimage:20210610&platform=linux/arm64" 200
+t GET "images/testimage:20210610/json" 200 \
.Architecture=arm64
# Make sure that new images are pulled
