ci(travis): run salt-lint for .tst files as well

.travis.yml

@@ -50,7 +50,7 @@ jobs:
       script:
         # Install and run `salt-lint`
         - pip install --user salt-lint
-        - git ls-files | grep '\.sls$\|\.jinja$\|\.j2$\|\.tmpl$'
+        - git ls-files | grep '\.sls$\|\.jinja$\|\.j2$\|\.tmpl$\|\.tst$'
                        | xargs -I {} salt-lint {}
         # Install and run `yamllint`
         # Need at least `v1.17.0` for the `yaml-files` setting