Fix ABSPATH detection on Windows

[andrewfrench]

Describe the bug
WordPress in a subdirectory on Windows causes the ABSPATH value to erroneously contain a . character instead of the correct path.

To Reproduce
On Windows,

• Create a WordPress project where WP is located in a directory like: docroot/wp/ and has no existing wp-config.php
• ddev config
• docroot/wp-config.php has an incorrect ABSPATH value

Expected behavior
ABSPATH should be set to something like dir(__FILE__) . "/wp", not dir(__FILE__) . "/."

Version and configuration information (please complete the following information):

• Windows
• ddev v1.3.0

[rfay]

We might as well add the test stanza for WP-in-subdir (#1167) in this one. Once again we're skunked by not having Windows tests for a new feature.