Avoid memory allocation in some functions from 'ltests.c'

To allow their use in memory tests, some functions in 'ltests.c'
should never allocate memory. To avoid this allocation, the
library registers the strings used for status codes, and keeps
the variable '_WARN' always defined (with false instead of nil).

testes/gc.lua

@@ -372,7 +372,7 @@ if T then
   warn("@on"); warn("@store")
   collectgarbage()
   assert(string.find(_WARN, "error in __gc metamethod"))
-  assert(string.match(_WARN, "@(.-)@") == "expected"); _WARN = nil
+  assert(string.match(_WARN, "@(.-)@") == "expected"); _WARN = false
   for i = 8, 10 do assert(s[i]) end
 
   for i = 1, 5 do
@@ -481,7 +481,7 @@ if T then
   u = setmetatable({}, {__gc = function () error "@expected error" end})
   u = nil
   collectgarbage()
-  assert(string.find(_WARN, "@expected error")); _WARN = nil
+  assert(string.find(_WARN, "@expected error")); _WARN = false
   warn("@normal")
 end
 
@@ -657,14 +657,14 @@ if T then
     n = n + 1
     assert(n == o[1])
     if n == 1 then
-      _WARN = nil
+      _WARN = false
     elseif n == 2 then
       assert(find(_WARN, "@expected warning"))
       lastmsg = _WARN    -- get message from previous error (first 'o')
     else
       assert(lastmsg == _WARN)  -- subsequent error messages are equal
     end
-    warn("@store"); _WARN = nil
+    warn("@store"); _WARN = false
     error"@expected warning"
   end}
   for i = 10, 1, -1 do