Add upgrade tests to test main against previous release

This commit adds the upgrade tests for testing main against the e2e
tests from the previous release to prevent regression. Incompatible
changes made in the current release cycle are expected to be catched.

test/e2e-common.sh

@@ -169,3 +169,9 @@ function delete_pipeline_resources() {
 function delete_resolvers_resources() {
   kubectl delete --ignore-not-found=true resolutionrequests.resolution.tekton.dev --all
 }
+
+function get_tests_from_release() {
+  echo ">> Retrieving tests from release-$1"
+  RELEASE_BRANCH="release-$1"
+  git checkout $RELEASE_BRANCH
+}

test/e2e-tests-upgrade.sh

@@ -24,6 +24,10 @@
 # Scenario 2: install the previous release, create the pipelines and tasks, upgrade
 # to the current release, and validate whether the Tekton pipeline works.
 
+# Scenario 3: install the current pipelines, use the integration tests from the previous release(s)
+# to prevent regression. Incompatible changes made in the current release cycle are expected to fail
+# and will be skipped after failing once.
+
 source $(git rev-parse --show-toplevel)/test/e2e-common.sh
 RELEASES='https://github.com/tektoncd/pipeline/releases/latest'
 PREVIOUS_PIPELINE_VERSION=$(curl -L -s -H 'Accept: application/json' $RELEASES |
@@ -85,6 +89,18 @@ go_test_e2e -timeout=20m ./test || failed=1
 # the upgrade.
 go_test_e2e -tags=examples -timeout=20m ./test/ || failed=1
 
+# Testing main against the latest release to detect regression.
+# TODO: #5782 1) add previous LTS and supported releases
+# 2) skip backward incompatible tests that failed once to keep tests running
+header "Testing main against the latest release to detect regression"
+get_tests_from_release $PREVIOUS_PIPELINE_VERSION
+
+# Run the integration tests.
+go_test_e2e -timeout=20m ./test || failed=1
+
+# Run the post-integration tests.
+go_test_e2e -tags=examples -timeout=20m ./test/ || failed=1
+
 (( failed )) && fail_test
 
 success