patch 8.2.0429: no warning when test checks for option that never exists

Problem:    No warning when test checks for option that never exists.
Solution:   In tests check that the option can exist.

src/testdir/check.vim

@@ -17,6 +17,9 @@ endfunc
 " Command to check for the presence of a working option.
 command -nargs=1 CheckOption call CheckOption(<f-args>)
 func CheckOption(name)
+  if !exists('&' .. a:name)
+    throw 'Checking for non-existent option ' .. a:name
+  endif
   if !exists('+' .. a:name)
     throw 'Skipped: ' .. a:name .. ' option not supported'
   endif

src/version.c

@@ -738,6 +738,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    429,
 /**/
     428,
 /**/