Document E2E tests on Pipeline for Merged Results

doc/development/testing_guide/end_to_end/index.md

@@ -88,6 +88,27 @@ subgraph "gitlab-qa pipeline"
 Please note, we plan to [add more specific information](https://gitlab.com/gitlab-org/quality/team-tasks/issues/156)
 about the tests included in each job/scenario that runs in `gitlab-qa`.
 
+#### With Pipeline for Merged Results
+
+In a Pipeline for Merged Results, the pipeline runs on a new ref that contains the merge result of the source and target branch.
+However, this ref is not available to the `gitlab-qa` pipeline.
+
+For this reason, the end-to-end tests on a Pipeline for Merged Results would use the head of the merge request source branch.
+
+```mermaid
+graph LR
+
+A["a1b1c1 - branch HEAD (CI_MERGE_REQUEST_SOURCE_BRANCH_SHA)"]
+B["x1y1z1 - master HEAD"]
+C["d1e1f1 - merged results (CI_COMMIT_SHA)"]
+
+A --> C
+B --> C
+
+A --> E["E2E tests"]
+C --> D["Pipeline for merged results"]
+ ```
+
 ##### Running custom tests
 
 The [existing scenarios](https://gitlab.com/gitlab-org/gitlab-qa/blob/master/docs/what_tests_can_be_run.md)

scripts/trigger-build

@@ -84,7 +84,7 @@ module Trigger
     end
 
     def base_variables
-      # Use CI_MERGE_REQUEST_SOURCE_BRANCH_SHA for omnibus checkouts
+      # Use CI_MERGE_REQUEST_SOURCE_BRANCH_SHA for omnibus checkouts due to pipeline for merged results
       {
         'GITLAB_REF_SLUG' => ENV['CI_COMMIT_TAG'] ? ENV['CI_COMMIT_REF_NAME'] : ENV['CI_COMMIT_REF_SLUG'],
         'TRIGGERED_USER' => ENV['TRIGGERED_USER'] || ENV['GITLAB_USER_NAME'],
@@ -126,7 +126,7 @@ module Trigger
     end
 
     def extra_variables
-      # Use CI_MERGE_REQUEST_SOURCE_BRANCH_SHA for omnibus checkouts
+      # Use CI_MERGE_REQUEST_SOURCE_BRANCH_SHA for omnibus checkouts due to pipeline for merged results
       {
         'GITLAB_VERSION' => ENV['CI_MERGE_REQUEST_SOURCE_BRANCH_SHA'],
         'ALTERNATIVE_SOURCES' => 'true',