docs: add kratman as a contributor for test (#3363)

* docs: update README.md [skip ci]

* docs: update .all-contributorsrc [skip ci]

---------

Co-authored-by: allcontributors[bot] <46447321+allcontributors[bot]@users.noreply.github.com>

.all-contributorsrc

@@ -694,7 +694,8 @@
         "doc",
         "infra",
         "bug",
-        "code"
+        "code",
+        "test"
       ]
     }
   ],

README.md

@@ -264,7 +264,7 @@ Thanks goes to these wonderful people ([emoji key](https://allcontributors.org/d
       <td align="center" valign="top" width="14.28%"><a href="https://github.com/tommaull"><img src="https://avatars.githubusercontent.com/u/101814207?v=4?s=100" width="100px;" alt="Tom Maull"/><br /><sub><b>Tom Maull</b></sub></a><br /><a href="https://github.com/pybamm-team/PyBaMM/commits?author=tommaull" title="Code">💻</a> <a href="https://github.com/pybamm-team/PyBaMM/commits?author=tommaull" title="Tests">⚠️</a></td>
       <td align="center" valign="top" width="14.28%"><a href="https://github.com/ejfdickinson"><img src="https://avatars.githubusercontent.com/u/116663050?v=4?s=100" width="100px;" alt="ejfdickinson"/><br /><sub><b>ejfdickinson</b></sub></a><br /><a href="#ideas-ejfdickinson" title="Ideas, Planning, & Feedback">🤔</a> <a href="https://github.com/pybamm-team/PyBaMM/issues?q=author%3Aejfdickinson" title="Bug reports">🐛</a></td>
       <td align="center" valign="top" width="14.28%"><a href="https://github.com/bobonice"><img src="https://avatars.githubusercontent.com/u/22030806?v=4?s=100" width="100px;" alt="bobonice"/><br /><sub><b>bobonice</b></sub></a><br /><a href="https://github.com/pybamm-team/PyBaMM/issues?q=author%3Abobonice" title="Bug reports">🐛</a> <a href="https://github.com/pybamm-team/PyBaMM/commits?author=bobonice" title="Code">💻</a></td>
-      <td align="center" valign="top" width="14.28%"><a href="https://github.com/kratman"><img src="https://avatars.githubusercontent.com/u/10170302?v=4?s=100" width="100px;" alt="Eric G. Kratz"/><br /><sub><b>Eric G. Kratz</b></sub></a><br /><a href="https://github.com/pybamm-team/PyBaMM/commits?author=kratman" title="Documentation">📖</a> <a href="#infra-kratman" title="Infrastructure (Hosting, Build-Tools, etc)">🚇</a> <a href="https://github.com/pybamm-team/PyBaMM/issues?q=author%3Akratman" title="Bug reports">🐛</a> <a href="https://github.com/pybamm-team/PyBaMM/commits?author=kratman" title="Code">💻</a></td>
+      <td align="center" valign="top" width="14.28%"><a href="https://github.com/kratman"><img src="https://avatars.githubusercontent.com/u/10170302?v=4?s=100" width="100px;" alt="Eric G. Kratz"/><br /><sub><b>Eric G. Kratz</b></sub></a><br /><a href="https://github.com/pybamm-team/PyBaMM/commits?author=kratman" title="Documentation">📖</a> <a href="#infra-kratman" title="Infrastructure (Hosting, Build-Tools, etc)">🚇</a> <a href="https://github.com/pybamm-team/PyBaMM/issues?q=author%3Akratman" title="Bug reports">🐛</a> <a href="https://github.com/pybamm-team/PyBaMM/commits?author=kratman" title="Code">💻</a> <a href="https://github.com/pybamm-team/PyBaMM/commits?author=kratman" title="Tests">⚠️</a></td>
     </tr>
   </tbody>
 </table>