More generic pattern when testing 'string.format'

The result of 'string.format("%a", 0.0)' can have multiple zeros
after the dot.

testes/strings.lua

@@ -309,8 +309,8 @@ do print("testing 'format %a %A'")
     matchhexa(n)
   end
 
-  assert(string.find(string.format("%A", 0.0), "^0X0%.?0?P%+?0$"))
-  assert(string.find(string.format("%a", -0.0), "^%-0x0%.?0?p%+?0$"))
+  assert(string.find(string.format("%A", 0.0), "^0X0%.?0*P%+?0$"))
+  assert(string.find(string.format("%a", -0.0), "^%-0x0%.?0*p%+?0$"))
 
   if not _port then   -- test inf, -inf, NaN, and -0.0
     assert(string.find(string.format("%a", 1/0), "^inf"))