python39Packages.pytest-xdist: add hook to add flags

many of our packages have pytest-xdist in their inputs however
oftentimes the required flags aren't added to pytest flags
since we dont use tox and other frameworks

some examples of improvements
(my pc has ryzen 9 5900x)
hypothesis 11m:15s on hydra -> <1m on my pc
hypothesmith 8m on my pc -> <1m on my pc

the whole graph from pytest-xdist (tested by adding postPatch with true to xdist)
time nix build ".#python3Packages.hypothesmith"
19m:57s -> 9m:13s

pkgs/development/python-modules/pytest-xdist/default.nix

@@ -29,6 +29,12 @@ buildPythonPackage rec {
   checkInputs = [ pytestCheckHook filelock pexpect ];
   propagatedBuildInputs = [ execnet pytest-forked psutil ];
 
+  pytestFlagsArray = [
+    # pytest can already use xdist at this point
+    "--numprocesses=$NIX_BUILD_CORES"
+    "--forked"
+  ];
+
   # access file system
   disabledTests = [
     "test_distribution_rsyncdirs_example"
@@ -41,6 +47,8 @@ buildPythonPackage rec {
     "test_internal_errors_propagate_to_controller"
   ];
 
+  setupHook = ./setup-hook.sh;
+
   meta = with lib; {
     description = "Pytest xdist plugin for distributed testing and loop-on-failing modes";
     homepage = "https://github.com/pytest-dev/pytest-xdist";

pkgs/development/python-modules/pytest-xdist/setup-hook.sh

@@ -0,0 +1,7 @@
+pytestXdistHook() {
+    pytestFlagsArray+=("--numprocesses=$NIX_BUILD_CORES" "--forked" )
+}
+
+if [ -z "${dontUsePytestXdist-}" ] && [ -z "${dontUsePytestCheck-}" ]; then
+    addEnvHooks "$hostOffset" pytestXdistHook
+fi