patch 8.1.0479: failure when setting 'varsofttabstop' to end in a comma

Problem:    Failure when setting 'varsofttabstop' to end in a comma. (Ralf
            Schandl)
Solution:   Reject value with trailing command.  Add test for invalid values
            (closes #3544)
diff --git a/src/testdir/test_vartabs.vim b/src/testdir/test_vartabs.vim
index c43e178..c847095 100644
--- a/src/testdir/test_vartabs.vim
+++ b/src/testdir/test_vartabs.vim
@@ -5,11 +5,11 @@
 endif
 
 source view_util.vim
-function! s:compare_lines(expect, actual)
+func s:compare_lines(expect, actual)
   call assert_equal(join(a:expect, "\n"), join(a:actual, "\n"))
-endfunction
+endfunc
 
-func! Test_vartabs()
+func Test_vartabs()
   new
   %d
 
@@ -261,7 +261,7 @@
   bwipeout!
 endfunc
 
-func! Test_vartabs_linebreak()
+func Test_vartabs_linebreak()
   if winwidth(0) < 40
     return
   endif
@@ -296,3 +296,14 @@
   bw!
   set nolist listchars&vim
 endfunc
+
+func Test_vartabs_failures()
+  call assert_fails('set vts=8,')
+  call assert_fails('set vsts=8,')
+  call assert_fails('set vts=8,,8')
+  call assert_fails('set vsts=8,,8')
+  call assert_fails('set vts=8,,8,')
+  call assert_fails('set vsts=8,,8,')
+  call assert_fails('set vts=,8')
+  call assert_fails('set vsts=,8')
+endfunc