patch 8.1.1483: skipped tests are not properly listed

Problem:    Skipped tests are not properly listed.
Solution:   Throw a "Skipped" exception instead of using ":finish" or ":return".
diff --git a/src/testdir/test_channel.vim b/src/testdir/test_channel.vim
index 2f2b559..982a045 100644
--- a/src/testdir/test_channel.vim
+++ b/src/testdir/test_channel.vim
@@ -1,7 +1,7 @@
 " Test for channel functions.
 
 if !has('channel')
-  finish
+  throw 'Skipped: channel feature missing'
 endif
 
 source shared.vim
@@ -9,7 +9,7 @@
 let s:python = PythonProg()
 if s:python == ''
   " Can't run this test without Python.
-  finish
+  throw 'Skipped: Python command missing'
 endif
 
 " Uncomment the next line to see what happens. Output is in