patch 8.1.1472: add_termcap_entry() is not tested

Problem:    Add_termcap_entry() is not tested.
Solution:   Add a simple test.
diff --git a/src/testdir/test_termcodes.vim b/src/testdir/test_termcodes.vim
index d182761..d602ee5 100644
--- a/src/testdir/test_termcodes.vim
+++ b/src/testdir/test_termcodes.vim
@@ -626,7 +626,6 @@
 endfunc
 
 " This only checks if the sequence is recognized.
-" TODO: check that the values were parsed properly
 func Test_term_rgb_response()
   set t_RF=x
   set t_RB=y
@@ -712,3 +711,10 @@
 
   set t_RV=
 endfunc
+
+func Test_get_termcode()
+  let k1 = &t_k1
+  set t_k1=
+  set t_k1&
+  call assert_equal(k1, &t_k1)
+endfunc