avro-cpp: enable tests

Unit tests are built unconditionally by the build system.
Might as well run them, since they take very little time to execute.

pkgs/by-name/av/avro-cpp/package.nix

@@ -31,6 +31,8 @@ stdenv.mkDerivation rec {
     fmt
   ];
 
+  doCheck = true;
+
   meta = {
     description = "C++ library which implements parts of the Avro Specification";
     mainProgram = "avrogencpp";