patch 8.2.3109: check for $DISPLAY never fails

Problem:    Check for $DISPLAY never fails.
Solution:   Use eval().

src/testdir/check.vim

@@ -139,7 +139,7 @@ endfunc
 " Command to Check for an environment variable
 command -nargs=1 CheckEnv call CheckEnv(<f-args>)
 func CheckEnv(name)
-  if empty('$' .. a:name)
+  if empty(eval('$' .. a:name))
     throw 'Skipped: Environment variable ' .. a:name .. ' is not set'
   endif
 endfunc

src/version.c

@@ -755,6 +755,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    3109,
 /**/
     3108,
 /**/