Test: rework FMF test metadata and test discovery for integration tests

Do not execute test script in the test plan, but use TMT test discovery
instead.

Related: rhbz#2174845

Signed-off-by: Tomáš Hozza <thozza@redhat.com>

plans/integration-tests.fmf

@@ -1,9 +1,10 @@
-summary: Build simple image using osbuild
+summary: Run osbuild integration tests
 prepare:
     how: install
     package:
     - osbuild
+discover:
+    how: fmf
+    filter: tag:integration-test
 execute:
-    script: ./tests/scripts/build-images.sh ./tests/manifests
-    duration: 4h
-
+    how: tmt

tests/image-build/main.fmf

@@ -0,0 +1,4 @@
+summary: Build simple image using osbuild
+tag: integration-test
+duration: 1h
+test: ./build-images.sh ./manifests