Build/Test Tools: Call `wp_head()` and `wp_footer()` in the theme used during tests.

See #31550
Fixes #39988

Merges [40235] to the 4.7 branch.


git-svn-id: https://develop.svn.wordpress.org/branches/4.7@40236 602fd350-edb4-49c9-b593-d223f7449a82

tests/phpunit/data/themedir1/default/index.php

@@ -1,3 +1,5 @@
 <?php
 
-// Empty theme
+wp_head();
+
+wp_footer();