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_conceal.vim b/src/testdir/test_conceal.vim
index 66384e1..f114651 100644
--- a/src/testdir/test_conceal.vim
+++ b/src/testdir/test_conceal.vim
@@ -2,12 +2,12 @@
 " Also see test88.in (should be converted to a test function here).
 
 if !has('conceal')
-  finish
+  throw 'Skipped: conceal feature missing'
 endif
 
 source screendump.vim
 if !CanRunVimInTerminal()
-  finish
+  throw 'Skipped: cannot make screendumps'
 endif
 
 func Test_conceal_two_windows()