Upgrade bats-core to latest and fix background process handling in tests

[mengesb]

Context

PR #18 upgraded GitHub Actions but had to pin bats-core at v1.2.1 because newer versions (1.7+) hang on the test suite. The root cause is well-understood and the fix is straightforward.

Problem

bats-core 1.7+ changed behavior: it now waits for all background processes spawned during setup_file() to exit before considering a test file complete. Two test files start ec2-metadata-mock as a background process in setup_file() but never kill it in teardown_file():

• test/aws-credentials.bats
• test/vault-iam-auth.bats

This causes bats to hang indefinitely after all tests pass, waiting for the mock to exit.

Fix

Add pkill -f ec2-metadata-mock || true to teardown_file() in both files:

function teardown_file() {
  pkill -f ec2-metadata-mock || true
  unset EC2_METADATA_MOCK
}

Once that's in place, update .github/workflows/bats-unit.yml to install a modern bats version:

    - name: Setup BATS
      shell: bash
      run: |
        git clone --depth 1 --branch v1.11.1 https://github.com/bats-core/bats-core.git /tmp/bats-core
        sudo /tmp/bats-core/install.sh /usr/local

Verification

After making the changes:

1. Run bats test/requirements.bats to confirm bats is functional
2. Run each test file individually and confirm it exits cleanly: bats test/aws-credentials.bats
3. Run the full suite sequentially to confirm no port conflicts between files

Additional notes

• test/gcp-credentials.bats and test/vault-gce-auth.bats may have a similar pattern with the GCE metadata mock — check those too
• The 3>&- trick on the ec2-metadata-mock launch line was a workaround for older bats FD inheritance; it's no longer sufficient in 1.7+
• Current pinned version (v1.2.1) works but misses years of bats improvements (parallel test execution, better TAP output, etc.)