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/Makefile b/runtime/doc/Makefile
index 3db1190..4a1ea03 100644
--- a/runtime/doc/Makefile
+++ b/runtime/doc/Makefile
@@ -103,6 +103,7 @@
 	tagsrch.txt \
 	term.txt \
 	terminal.txt \
+	testing.txt \
 	textprop.txt \
 	tips.txt \
 	todo.txt \
@@ -241,6 +242,7 @@
 	tagsrch.html \
 	term.html \
 	terminal.html \
+	testing.html \
 	textprop.html \
 	tips.html \
 	todo.html \