patch 8.1.1190: has('vimscript-3') does not work

Problem:    has('vimscript-3') does not work.
Solution:   Add "vimscript-3" to the list of features.

src/testdir/test_eval_stuff.vim

@@ -126,6 +126,7 @@ endfunc
 
 scriptversion 2
 func Test_string_concat_scriptversion2()
+  call assert_true(has('vimscript-2'))
   let a = 'a'
   let b = 'b'
 
@@ -141,6 +142,7 @@ endfunc
 
 scriptversion 1
 func Test_string_concat_scriptversion1()
+  call assert_true(has('vimscript-1'))
   let a = 'a'
   let b = 'b'
 
@@ -156,6 +158,7 @@ endfunc
 
 scriptversion 3
 func Test_vvar_scriptversion3()
+  call assert_true(has('vimscript-3'))
   call assert_fails('echo version', 'E121:')
   call assert_false(exists('version'))
   let version = 1