patch 8.1.1634: terminal test fails when term_getansicolors() is missing

Problem:    Terminal test fails when term_getansicolors() is missing.
            Diff test fails without +rightleft.  (Dominique Pelle)
Solution:   Check if term_getansicolors() is supported. (closes #4597)
diff --git a/src/testdir/test_terminal.vim b/src/testdir/test_terminal.vim
index fa9eb1e..6a9a6aa 100644
--- a/src/testdir/test_terminal.vim
+++ b/src/testdir/test_terminal.vim
@@ -1434,6 +1434,9 @@
 endfunc
 
 func Test_terminal_ansicolors_default()
+  if !exists('*term_getansicolors')
+    throw 'Skipped: term_getansicolors() not supported'
+  endif
   let colors = [
 	\ '#000000', '#e00000',
 	\ '#00e000', '#e0e000',
@@ -1465,6 +1468,9 @@
 	\]
 
 func Test_terminal_ansicolors_global()
+  if !exists('*term_getansicolors')
+    throw 'Skipped: term_getansicolors() not supported'
+  endif
   let g:terminal_ansi_colors = reverse(copy(s:test_colors))
   let buf = Run_shell_in_terminal({})
   call assert_equal(g:terminal_ansi_colors, term_getansicolors(buf))
@@ -1476,6 +1482,9 @@
 endfunc
 
 func Test_terminal_ansicolors_func()
+  if !exists('*term_getansicolors')
+    throw 'Skipped: term_getansicolors() not supported'
+  endif
   let g:terminal_ansi_colors = reverse(copy(s:test_colors))
   let buf = Run_shell_in_terminal({'ansi_colors': s:test_colors})
   call assert_equal(s:test_colors, term_getansicolors(buf))