patch 8.2.3092: Vim9: builtin function test fails without +channel feature

Problem:    Vim9: builtin function test fails without the +channel feature.
Solution:   Check the +channel feature is supported. (Dominique Pellé,
            closes #8507)
diff --git a/src/testdir/test_vim9_builtin.vim b/src/testdir/test_vim9_builtin.vim
index 5a5b512..38bd827 100644
--- a/src/testdir/test_vim9_builtin.vim
+++ b/src/testdir/test_vim9_builtin.vim
@@ -1344,8 +1344,10 @@
 enddef
 
 def Test_prompt_getprompt()
-  CheckDefFailure(['prompt_getprompt([])'], 'E1013: Argument 1: type mismatch, expected string but got list<unknown>')
-  assert_equal('', prompt_getprompt('NonExistingBuf'))
+  if has('channel')
+    CheckDefFailure(['prompt_getprompt([])'], 'E1013: Argument 1: type mismatch, expected string but got list<unknown>')
+    assert_equal('', prompt_getprompt('NonExistingBuf'))
+  endif
 enddef
 
 def Test_rand()