patch 8.1.2118: termcodes test fails when $TERM is "dumb"

Problem:    Termcodes test fails when $TERM is "dumb".
Solution:   Skip the test. (James McCoy, closes #5019)
diff --git a/src/testdir/test_termcodes.vim b/src/testdir/test_termcodes.vim
index 96bbdbb..6174723 100644
--- a/src/testdir/test_termcodes.vim
+++ b/src/testdir/test_termcodes.vim
@@ -798,7 +798,11 @@
 endfunc
 
 func Test_get_termcode()
-  let k1 = &t_k1
+  try
+    let k1 = &t_k1
+  catch /E113/
+    throw 'Skipped: Unable to query termcodes'
+  endtry
   set t_k1=
   set t_k1&
   call assert_equal(k1, &t_k1)