patch 8.1.1839: insufficient info when test fails because of screen size

Problem:    Insufficient info when test fails because of screen size.
Solution:   Report the detected screen size.

src/testdir/runtest.vim

@@ -34,12 +34,14 @@ so small.vim
 
 " Check that the screen size is at least 24 x 80 characters.
 if &lines < 24 || &columns < 80 
-  let error = 'Screen size too small! Tests require at least 24 lines with 80 characters'
+  let error = 'Screen size too small! Tests require at least 24 lines with 80 characters, got ' .. &lines .. ' lines with ' .. &columns .. ' characters'
   echoerr error
   split test.log
   $put =error
   write
   split messages
+  call append(line('$'), '')
+  call append(line('$'), 'From ' . expand('%') . ':')
   call append(line('$'), error)
   write
   qa!
@@ -170,7 +172,7 @@ func RunTheTest(test)
     endtry
   endif
 
-  " Clear any autocommands
+  " Clear any autocommands and put back the catch-all for SwapExists.
   au!
   au SwapExists * call HandleSwapExists()
 

src/version.c

@@ -769,6 +769,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    1839,
 /**/
     1838,
 /**/