tests/ghci/scripts/T8172 is sensitive to drive name on Windows

The test fails when the checkout is not on drive C:, like this:

--- ./ghci/scripts/T8172.stdout-mingw32 2014-10-28 23:28:12.264410700 +0100
+++ ./ghci/scripts/T8172.run.stdout     2014-10-30 15:20:13.964595800 +0100
@@ -1,11 +1,11 @@
 current working directory:
-  C:\
+  K:\
 module import search paths:
   .
 current working directory:
-  C:\
+  K:\
 module import search paths: none
 current working directory:
-  C:\
+  K:\
 module import search paths:
   /