patch 8.1.1474: 'ttybuiltin' is not tested

Problem:    'ttybuiltin' is not tested.
Solution:   At least test that it doesn't break things.
diff --git a/src/testdir/test_termcodes.vim b/src/testdir/test_termcodes.vim
index d602ee5..9cfc649 100644
--- a/src/testdir/test_termcodes.vim
+++ b/src/testdir/test_termcodes.vim
@@ -717,4 +717,24 @@
   set t_k1=
   set t_k1&
   call assert_equal(k1, &t_k1)
+
+  " use external termcap first
+  set nottybuiltin
+  set t_k1=
+  set t_k1&
+  " when using external termcap may get something else, but it must not be
+  " empty, since we would fallback to the builtin one.
+  call assert_notequal('', &t_k1)
+
+  if &term =~ 'xterm'
+    " use internal termcap first
+    let term_save = &term
+    let &term = 'builtin_' .. &term
+    set t_k1=
+    set t_k1&
+    call assert_equal(k1, &t_k1)
+    let &term = term_save
+  endif
+
+  set ttybuiltin
 endfunc
diff --git a/src/version.c b/src/version.c
index cd138ab..a422cb0 100644
--- a/src/version.c
+++ b/src/version.c
@@ -768,6 +768,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    1474,
+/**/
     1473,
 /**/
     1472,