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_termcodes.vim b/src/testdir/test_termcodes.vim
index c6327db..475a09a 100644
--- a/src/testdir/test_termcodes.vim
+++ b/src/testdir/test_termcodes.vim
@@ -1,12 +1,9 @@
 " Tests for decoding escape sequences sent by the terminal.
 
 " This only works for Unix in a terminal
-if has('gui_running')
-  throw 'Skipped: does not work in the GUI'
-endif
-if !has('unix')
-  throw 'Skipped: not on Unix'
-endif
+source check.vim
+CheckNotGui
+CheckUnix
 
 source shared.vim