patch 8.2.3780: ":cd" works differently on MS-Windows

Problem:    ":cd" works differently on MS-Windows.
Solution:   Add the 'cdhome' option. (closes #9324)
diff --git a/src/testdir/runtest.vim b/src/testdir/runtest.vim
index 28e23c0..912bc20 100644
--- a/src/testdir/runtest.vim
+++ b/src/testdir/runtest.vim
@@ -77,6 +77,9 @@
   let s:start_time = reltime()
 endif
 
+" Always use forward slashes.
+set shellslash
+
 " Common with all tests on all systems.
 source setup.vim
 
@@ -128,9 +131,6 @@
   source $VIMRUNTIME/menu.vim
 endif
 
-" Always use forward slashes.
-set shellslash
-
 let s:srcdir = expand('%:p:h:h')
 
 if has('win32')