Update tiny containers test cases names

Update the test cases names so they match the names specified
in the lava job file.

Signed-off-by: Luis Araujo <luis.araujo@collabora.co.uk>

test-cases/check-tiny-system-containers.yaml → test-cases/check-tiny-system-container.yaml

 metadata:
-  name: check-tiny-system-containers
+  name: check-tiny-system-container
   format: "Apertis Test Definition 1.0"
   image-type: tiny-lxc
   image-arch: any

test-cases/check-tiny-user-containers.yaml → test-cases/check-tiny-user-container.yaml

 metadata:
-  name: check-tiny-user-containers
+  name: check-tiny-user-container
   format: "Apertis Test Definition 1.0"
   image-type: tiny-lxc
   image-arch: any