Add unused reachability test config option

sbws/config.default.ini

 [paths]
-[helpers]
 [server.passwords]
 
+[helpers]
+# Test the reachability of our configured helpers after this many seconds pass
+reachability_test_every = 300
+
 [general]
 # Days into the past that measurements are considered valid
 data_period = 5

sbws/util/config.py

@@ -243,8 +243,14 @@ def _validate_helpers(conf):
     err_tmpl = Template('$sec/$key ($val): $e')
     addtional_helper_sections = []
     for key in section.keys():
-        valid, error_msg = _validate_boolean(section, key)
         value = section[key]
+        if key == 'reachability_test_every':
+            valid, error_msg = _validate_int(section, key, minimum=1)
+            if not valid:
+                errors.append(err_tmpl.substitute(
+                    sec=sec, key=key, val=value, e=error_msg))
+            continue
+        valid, error_msg = _validate_boolean(section, key)
         if not valid:
             errors.append(err_tmpl.substitute(
                 sec=sec, key=key, val=value, e=error_msg))