test(pr create): add AT for remote:branch syntax

acceptance/testdata/pr/pr-create-respects-remote-colon-branch-syntax.txtar

@@ -0,0 +1,46 @@
+# skip 'it creates a fork owned by the user running the test'
+
+# Setup environment variables used for testscript
+env REPO=${SCRIPT_NAME}-${RANDOM_STRING}
+env FORK=${REPO}-fork
+
+# Use gh as a credential helper
+exec gh auth setup-git
+
+# Get the current username for the fork owner
+exec gh api user --jq .login
+stdout2env USER
+
+# Create a repository to act as upstream with a file so it has a default branch
+exec gh repo create ${ORG}/${REPO} --add-readme --private
+
+# Defer repo cleanup of upstream
+defer gh repo delete --yes ${ORG}/${REPO}
+
+# Create a user fork of repository. This will be owned by USER.
+exec gh repo fork ${ORG}/${REPO} --fork-name ${FORK}
+
+# Defer repo cleanup of fork
+defer gh repo delete --yes ${USER}/${FORK}
+sleep 5
+exec gh repo view ${USER}/${FORK} --json id --jq '.id'
+stdout2env FORK_ID
+
+# Clone the repo
+exec gh repo clone ${USER}/${FORK}
+cd ${FORK}
+
+# Prepare a branch where changes are pulled from the upstream default branch but pushed to fork
+exec git checkout -b feature-branch upstream/main
+exec git config remote.pushDefault origin
+exec git config unset remote.upstream.gh-resolved
+exec git commit --allow-empty -m 'Empty Commit'
+exec git push
+
+# Create the PR spanning upstream and fork repositories
+exec gh pr create --title 'Feature Title' --body 'Feature Body' --head ${USER}:feature-branch
+stdout https://${GH_HOST}/${ORG}/${REPO}/pull/1
+
+# Assert that the PR was created with the correct head repository and refs
+exec gh pr view --json headRefName,headRepository,baseRefName,isCrossRepository
+stdout {"baseRefName":"main","headRefName":"feature-branch","headRepository":{"id":"${FORK_ID}","name":"${FORK}"},"isCrossRepository":true}