bugfix: Wipe tests/zones before generation

It is possible for timezones to disappear (well, unlikely but still),
and in that case there will be leftover tests for gone zones under
tests/zones. So wipe tests/zones clean when generating (latest) tests.
For versioned tests, the tests go under temp, which is wiped by default
already.

Gruntfile.js

@@ -43,7 +43,8 @@ module.exports = function(grunt) {
 		},
 
 		clean: {
-			data: ['temp']
+			data: ['temp'],
+			tests: ['tests/zones'],
 		},
 
 		exec: {

tasks/data.js

@@ -6,6 +6,9 @@ module.exports = function (grunt) {
 
 		if (!grunt.option('dirty-run')) {
 			grunt.task.run('clean:data');
+			if (version === 'latest') {
+				grunt.task.run('clean:tests');
+			}
 		}
 
 		grunt.task.run([