spigot: add a simple test

pkgs/by-name/sp/spigot/package.nix

···
 { lib
 , stdenv
 , fetchurl
+, callPackage
 , cmake
 , gmp
 , halibut
···
   outputs = [ "out" "man" ];
 
   strictDeps = true;
+
+  passthru.tests = {
+    approximation = callPackage ./tests/approximation.nix {
+      spigot = finalAttrs.finalPackage;
+    };
+  };
 
   meta = {
     homepage = "https://www.chiark.greenend.org.uk/~sgtatham/spigot/";

pkgs/by-name/sp/spigot/tests/approximation.nix

···
+{ lib
+, stdenv
+, spigot
+}:
+
+stdenv.mkDerivation {
+  pname = "spigot-approximation";
+  inherit (spigot) version;
+
+  nativeBuildInputs = [ spigot ];
+
+  dontInstall = true;
+
+  buildCommand = ''
+    [ "$(spigot -b2 -d32 '(pi/1-355/113)')" = "-0.00000000000000000000010001111001" ]
+    [ "$(spigot -b2 -d32 '(e/1-1457/536)')" = "-0.00000000000000000001110101101011" ]
+    touch $out
+  '';
+
+  meta.timeout = 10;
+}