Coveralls coverage is zero

Hello, thanks for creating this template it helps a lot to start plugin development.

In `test` GitHub job you provide section to evaluate tests coverage and post it to coveralls.io. After building plugin coverage calculated and posted, but coveralls shows it as 0%.

**Steps to reproduce:**
1. Generate repo from this template
2. Update `.github/workflows/test.yml` by removing Kong Enterprice versions
3. Create an account on coveralls.io and add your new repo
4. Kick off GitHub test action
5. Observe coverage in your repo on coveralls.io

**Expected result:**
step (4): Build successfull
step (5): Coverage greater than 0% shown

**Actual result:**
step (5): Coverage 0% shown

Going through my steps I've created repo https://github.com/seprokof/kong-plugin , added repo to coveralls.io https://coveralls.io/github/seprokof/kong-plugin and build is https://github.com/seprokof/kong-plugin/actions/runs/5438212946 . I've added little bit of debug info but outputting content of coverage files during the build.

I've checked source of `pongo.sh` from `kong-pongo` and saw that you just forward `--coverage` argument to `busted` and saw your notice that coverage only calculated for unit tests, but based on content of coverage files I expect to see something greater than zero on coveralls.io.

I detected the only suspicious thing - if you enable verbose output of `luacov-coveralls` by adding `-v` switch you will see `source_files: 0`.

Start with .github/workflows/test.yml and inspect how pongo.sh passes coverage to busted and how luacov-coveralls consumes the generated coverage files. Reproduce the workflow from the linked example repository, enable the mentioned verbose output, and trace why source_files is 0. Done means the GitHub test job still succeeds and the resulting Coveralls report shows non-zero coverage.

Written by the indexing model from the issue text.