dentarg/postgres is an composite run steps action that starts PostgreSQL in the GitHub Actions VM and creates a user and a database. You can then connect to the database using the address found in environment variable POSTGRES_URL.

jobs:
  build:
    runs-on: ubuntu-latest
    steps:
    - uses: dentarg/postgres@v1
    - run: <your test command>
      env:
        DATABASE_URL: "${{ env.POSTGRES_URL }}" # or use POSTGRES_URL directly

See the GitHub Actions Virtual Environments repo to understand what version of PostgreSQL will be used.