Avoid excessive name pollution in test files

Test files are more polite regarding the use of globals when locals
would do, and when globals are necessary deleting them after use.

testes/files.lua

@@ -507,15 +507,17 @@ load((io.lines(file, 1)))()
 assert(_G.X == 4)
 load((io.lines(file, 3)))()
 assert(_G.X == 8)
+_G.X = nil
 
 print('+')
 
 local x1 = "string\n\n\\com \"\"''coisas [[estranhas]] ]]'"
 io.output(file)
-assert(io.write(string.format("x2 = %q\n-- comment without ending EOS", x1)))
+assert(io.write(string.format("X2 = %q\n-- comment without ending EOS", x1)))
 io.close()
 assert(loadfile(file))()
-assert(x1 == x2)
+assert(x1 == _G.X2)
+_G.X2 = nil
 print('+')
 assert(os.remove(file))
 assert(not os.remove(file))