Revert "[tools] Disable some multiprocessing tests (#24892)"

This reverts commit b2579d4e08a237fd1e38942ead6303495f3fbf07.

Fixes #24880.
diff --git a/tools/serve/test_functional.py b/tools/serve/test_functional.py
index 5a918c2..894479d 100644
--- a/tools/serve/test_functional.py
+++ b/tools/serve/test_functional.py
@@ -9,7 +9,6 @@
     import Queue as queue  # noqa: N813
 except ImportError:
     import queue
-import sys
 import tempfile
 import threading
 
@@ -46,8 +45,6 @@
     os.remove(name)
 
 
-@pytest.mark.skipif(sys.version_info >= (3, 8) and sys.platform == 'darwin',
-                    reason="multiprocessing test hangs in Python 3.8 on macOS (#24880)")
 def test_subprocess_exit(server_subprocesses, tempfile_name):
     timeout = 30
 
diff --git a/tools/wpt/tests/test_wpt.py b/tools/wpt/tests/test_wpt.py
index 69a649a..ee338a3 100644
--- a/tools/wpt/tests/test_wpt.py
+++ b/tools/wpt/tests/test_wpt.py
@@ -184,8 +184,6 @@
 
 @pytest.mark.slow
 @pytest.mark.remote_network
-@pytest.mark.skipif(sys.version_info >= (3, 8) and sys.platform == 'darwin',
-                    reason="multiprocessing test hangs in Python 3.8 on macOS (#24880)")
 def test_run_failing_test():
     """Failing tests should be reported with a non-zero exit status unless the
     `--no-fail-on-unexpected` option has been specified."""
@@ -208,8 +206,6 @@
 
 @pytest.mark.slow
 @pytest.mark.remote_network
-@pytest.mark.skipif(sys.version_info >= (3, 8) and sys.platform == 'darwin',
-                    reason="multiprocessing test hangs in Python 3.8 on macOS (#24880)")
 def test_run_verify_unstable(temp_test):
     """Unstable tests should be reported with a non-zero exit status. Stable
     tests should be reported with a zero exit status."""
diff --git a/tools/wptserve/tests/functional/test_stash.py b/tools/wptserve/tests/functional/test_stash.py
index 0b960c1..1c4c9ad 100644
--- a/tools/wptserve/tests/functional/test_stash.py
+++ b/tools/wptserve/tests/functional/test_stash.py
@@ -1,4 +1,3 @@
-import sys
 import unittest
 import uuid
 
@@ -10,8 +9,6 @@
 from .base import TestUsingServer
 
 
-@pytest.mark.skipif(sys.version_info >= (3, 8) and sys.platform == 'darwin',
-                    reason="multiprocessing test hangs in Python 3.8 on macOS (#24880)")
 class TestResponseSetCookie(TestUsingServer):
     def run(self, result=None):
         with StashServer(None, authkey=str(uuid.uuid4())):