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_timers.vim b/src/testdir/test_timers.vim
index 42fe415..367e46f 100644
--- a/src/testdir/test_timers.vim
+++ b/src/testdir/test_timers.vim
@@ -239,9 +239,9 @@
 endfunc
 
 func Test_peek_and_get_char()
-  if !has('unix') && !has('gui_running')
-    return
-  endif
+  CheckUnix
+  CheckGui
+
   call timer_start(0, 'FeedAndPeek')
   let intr = timer_start(100, 'Interrupt')
   let c = getchar()
@@ -251,8 +251,7 @@
 
 func Test_getchar_zero()
   if has('win32') && !has('gui_running')
-    " Console: no low-level input
-    return
+    throw 'Skipped: cannot get low-level input'
   endif
 
   " Measure the elapsed time to avoid a hang when it fails.