refactor(saltcheck): relocate .tst files according to 1:1 .sls files

* https://github.com/saltstack-formulas/cron-formula/pull/4#issuecomment-544140377

cron/package/saltcheck-tests/install.tst

@@ -0,0 +1,13 @@
+# -*- coding: utf-8 -*-
+# vim: ft=sls
+
+{%- set package_name = 'cronie' %}
+{%- if grains.os_family in ['Debian'] %}
+{%-   set package_name = 'cron' %}
+{%- endif %}
+
+verify_cron.package:
+  module_and_function: pkg.version
+  args:
+    - {{ package_name }}
+  assertion: assertNotEmpty