fix(gh-actions-output): fixed trailing newline to match GITHUB_OUTPUT format

[codejedi365]

Purpose

Resolve #884 bug of a missing trailing newline which is expected by GitHub Actions

Rationale

The bug existed because str.join() only inserts a value between elements which means the last element did not receive a newline.

How I tested

Refactored the existing tests to more visually define the exact output value that should be placed in the output file. The tests now properly detect when the newline is missing.

How to verify

git fetch origin pull/885/head:pr-885

# checkout the PR as a detached HEAD state with only the unit tests
git checkout pr-885~1 --detach

# execute unit-tests 
pytest tests/unit/semantic_release/cli/test_github_actions_output.py

# update to the HEAD of the PR (with fixes)
git merge --ff-only pr-885

# run pytest again to see success
pytest tests/unit/semantic_release/cli/test_github_actions_output.py

# run full suite for verification
pytest