patch 9.0.0013: reproducing memory access errors can be difficult

Problem:    Reproducing memory access errors can be difficult.
Solution:   When testing, copy each line to allocated memory, so that valgrind
            can detect accessing memory before and/or after it.  Fix uncovered
            problems.

runtime/doc/testing.txt

@@ -268,6 +268,9 @@ test_override({name}, {val})				*test_override()*
 		Current supported values for {name} are:
 
 		{name}	     effect when {val} is non-zero ~
+		alloc_lines  make a copy of every buffer line into allocated
+			     memory, so that memory access errors can be found
+			     by valgrind
 		autoload     `import autoload` will load the script right
 			     away, not postponed until an item is used
 		char_avail   disable the char_avail() function
@@ -287,7 +290,8 @@ test_override({name}, {val})				*test_override()*
 		uptime 	     overrules sysinfo.uptime
 		vterm_title  setting the window title by a job running in a
 			     terminal window
-		ALL	     clear all overrides ({val} is not used)
+		ALL	     clear all overrides, except alloc_lines ({val} is
+			     not used)
 
 		"starting" is to be used when a test should behave like
 		startup was done.  Since the tests are run by sourcing a