patch 8.1.1826: tests use hand coded feature and option checks

Problem:    Tests use hand coded feature and option checks.
Solution:   Use the commands from check.vim in more tests.
diff --git a/src/testdir/test_terminal.vim b/src/testdir/test_terminal.vim
index 6a9a6aa..5496b63 100644
--- a/src/testdir/test_terminal.vim
+++ b/src/testdir/test_terminal.vim
@@ -1788,9 +1788,7 @@
 
 " must be nearly the last, we can't go back from GUI to terminal
 func Test_zz1_terminal_in_gui()
-  if !CanRunGui()
-    return
-  endif
+  CheckCanRunGui
 
   " Ignore the "failed to create input context" error.
   call test_ignore_error('E285:')
@@ -1810,9 +1808,7 @@
 endfunc
 
 func Test_zz2_terminal_guioptions_bang()
-  if !has('gui_running')
-    return
-  endif
+  CheckGui
   set guioptions+=!
 
   let filename = 'Xtestscript'