difftest: document nextest requirement

Author: Firestar99

default.nix

@@ -12,6 +12,7 @@ in with pkgs; stdenv.mkDerivation rec {
   SSL_CERT_FILE = "${cacert}/etc/ssl/certs/ca-bundle.crt";
 
   nativeBuildInputs = [ rustup ];
+  buildInputs = [ cargo-nextest ];
 
   # Runtime dependencies (for the example runners).
   LD_LIBRARY_PATH = with xorg; lib.makeLibraryPath [

tests/difftests/README.md

@@ -160,6 +160,12 @@ config.write_metadata(&metadata)?;
 
 ## Running Tests
 
+### Install cargo-nextest:
+
+```sh
+cargo install cargo-nextest
+```
+
 ### Run all difftests:
 
 ```sh