patch 8.1.1726: the eval.txt help file is too big

Problem:    The eval.txt help file is too big.
Solution:   Split off testing support to testing.txt.  Move function details
            to where the functionality is explained.
diff --git a/runtime/doc/help.txt b/runtime/doc/help.txt
index ef5394d..4550a5c 100644
--- a/runtime/doc/help.txt
+++ b/runtime/doc/help.txt
@@ -1,4 +1,4 @@
-*help.txt*	For Vim version 8.1.  Last change: 2019 May 12
+*help.txt*	For Vim version 8.1.  Last change: 2019 Jul 21
 
 			VIM - main help file
 									 k
@@ -139,6 +139,7 @@
 |fold.txt|	hide (fold) ranges of lines
 
 Special issues ~
+|testing.txt|	testing Vim and Vim scripts
 |print.txt|	printing
 |remote.txt|	using Vim as a server or client
 |term.txt|	using different terminals and mice