cpython: include test.support and test.regrtest

test.{support, regrtest} are the internal packages cpython
developers use to write tests.
Although they are not public and the API may change/break
some developers use these packages to write tests for their
(3rd party) software.
The derivations for cpython now only remove the actual tests
but leave the packages in place that are used to write them.

Discussion: NixOS#28540

pkgs/development/interpreters/python/cpython/2.7/default.nix

@@ -162,7 +162,8 @@ in stdenv.mkDerivation {
         # needed for some packages, especially packages that backport
         # functionality to 2.x from 3.x
         for item in $out/lib/python${majorVersion}/test/*; do
-          if [[ "$item" != */test_support.py* ]]; then
+          if [[ "$item" != */test_support.py*
+             && "$item" != */regrtest.py* ]]; then
             rm -rf "$item"
           else
             echo $item

pkgs/development/interpreters/python/cpython/3.4/default.nix

@@ -111,7 +111,10 @@ in stdenv.mkDerivation {
     # needed for some packages, especially packages that backport functionality
     # to 2.x from 3.x
     for item in $out/lib/python${majorVersion}/test/*; do
-      if [[ "$item" != */test_support.py* ]]; then
+      if [[ "$item" != */test_support.py*
+         && "$item" != */test/support
+         && "$item" != */test/libregrtest
+         && "$item" != */test/regrtest.py* ]]; then
         rm -rf "$item"
       else
         echo $item

pkgs/development/interpreters/python/cpython/3.5/default.nix

@@ -105,7 +105,10 @@ in stdenv.mkDerivation {
     # needed for some packages, especially packages that backport functionality
     # to 2.x from 3.x
     for item in $out/lib/python${majorVersion}/test/*; do
-      if [[ "$item" != */test_support.py* ]]; then
+      if [[ "$item" != */test_support.py*
+         && "$item" != */test/support
+         && "$item" != */test/libregrtest
+         && "$item" != */test/regrtest.py* ]]; then
         rm -rf "$item"
       else
         echo $item

pkgs/development/interpreters/python/cpython/3.6/default.nix

@@ -105,7 +105,10 @@ in stdenv.mkDerivation {
     # needed for some packages, especially packages that backport functionality
     # to 2.x from 3.x
     for item in $out/lib/python${majorVersion}/test/*; do
-      if [[ "$item" != */test_support.py* ]]; then
+      if [[ "$item" != */test_support.py*
+         && "$item" != */test/support
+         && "$item" != */test/libregrtest
+         && "$item" != */test/regrtest.py* ]]; then
         rm -rf "$item"
       else
         echo $item