patch 9.0.0852: crypt test is skipped if xxd is not found

Problem:    Crypt test is skipped if xxd is not found.
Solution:   Find xxd where it was supposed to be build.

src/testdir/test_crypt.vim

@@ -3,11 +3,20 @@
 source check.vim
 CheckFeature cryptv
 
-let s:xxd_cmd = ''
-if empty($XXDPROG) && executable('..\xxd\xxd.exe')
-  let s:xxd_cmd = '..\xxd\xxd.exe'
-elseif !empty($XXDPROG) && executable($XXDPROG)
+" Use the xxd command from:
+" 1: $XXDPROG if set and it is executable
+" 2: the ../xxd directory if the executable is found there
+if !empty($XXDPROG) && executable($XXDPROG)
   let s:xxd_cmd = $XXDPROG
+elseif executable('..\xxd\xxd.exe')
+  " we're on MS-Windows
+  let s:xxd_cmd = '..\xxd\xxd.exe'
+elseif executable('../xxd/xxd')
+  " we're on something like Unix
+  let s:xxd_cmd = '../xxd/xxd'
+else
+  " looks like xxd wasn't build (yet)
+  let s:xxd_cmd = ''
 endif
 
 func Common_head_only(text)

src/version.c

@@ -695,6 +695,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    852,
 /**/
     851,
 /**/