patch 8.1.1524: tests are silently skipped

Problem:    Tests are silently skipped.
Solution:   Throw an exception for skipped tests in more places.
diff --git a/src/testdir/shared.vim b/src/testdir/shared.vim
index 4546be7..ae023a7 100644
--- a/src/testdir/shared.vim
+++ b/src/testdir/shared.vim
@@ -330,7 +330,7 @@
 
 " Get line "lnum" as displayed on the screen.
 " Trailing white space is trimmed.
-func! Screenline(lnum)
+func Screenline(lnum)
   let chars = []
   for c in range(1, winwidth(0))
     call add(chars, nr2char(screenchar(a:lnum, c)))