Merge branch 'js/default-branch-name-adjust-t5515'

Prepare a test script to transition of the default branch name to
'main'.

* js/default-branch-name-adjust-t5515:
  t5515: use `main` as the name of the main branch for testing (conclusion)
  t5515: use `main` as the name of the main branch for testing (part 3)
  t5515: use `main` as the name of the main branch for testing (part 2)
  t5515: use `main` as the name of the main branch for testing (part 1)