Adds testing

[zgosalvez]

Potentially resolves #10 and #11.

[nackerman-nydig]

Didn't pull down and test, but just giving it an "eye-ball pass" looks good

[xunleii]

@zgosalvez I have this following error when I run your tests:

 FAIL  test/fail.test.js
  ● action has empty error

    expect(received).toContain(expected) // indexOf

    Expected substring: "Cannot read property 'jobs' of null"
    Received string:    "::debug::followSymbolicLinks 'true'
    ::debug::implicitDescendants 'true'
    ::debug::omitBrokenSymbolicLinks 'true'
    ::debug::Search path '***/github-actions-ensure-sha-pinned-actions/test/stub/empty'
    ::error::Cannot read properties of null (reading 'jobs')
    "

      21 |         const result = error.stdout.toString();
      22 |
    > 23 |         jest.expect(result).toContain('Cannot read property \'jobs\' of null');
         |                             ^
      24 |         jest.expect(result).not.toContain('No issues were found.');
      25 |     }
      26 | });

      at Object.<anonymous> (test/fail.test.js:23:29)

 PASS  test/pass.test.js

Test Suites: 1 failed, 1 passed, 2 total
Tests:       1 failed, 2 passed, 3 total
Snapshots:   0 total
Time:        0.441 s, estimated 1 s