gh-84461: Fix parallel testing on WebAssembly

Emscripten and WASI have stubbed getpid() syscall that always returns
the same value. Use time and random value instead of pid to create a
unique working directory.

Author: tiran

Lib/test/libregrtest/main.py

@@ -628,7 +628,12 @@ def create_temp_dir(self):
         # Define a writable temp dir that will be used as cwd while running
         # the tests. The name of the dir includes the pid to allow parallel
         # testing (see the -j option).
-        pid = os.getpid()
+        # Emscripten and WASI have stubbed getpid(), Emscripten has only
+        # milisecond clock resolution. Use time_ns() + randint() instead.
+        if sys.platform in {"emscripten", "wasi"}:
+            pid = time.time_ns() + random.randint(0, 1000000)
+        else:
+            pid = os.getpid()
         if self.worker_test_name is not None:
             test_cwd = 'test_python_worker_{}'.format(pid)
         else:

Tools/scripts/run_tests.py

@@ -63,9 +63,9 @@ def main(regrtest_args):
         args.append('-n')         # Silence alerts under Windows
     if not any(is_multiprocess_flag(arg) for arg in regrtest_args):
         if cross_compile and hostrunner:
-            # For now use only one core for cross-compiled builds;
+            # For now use only two cores for cross-compiled builds;
             # hostrunner can be expensive.
-            args.extend(['-j', '1'])
+            args.extend(['-j', '2'])
         else:
             args.extend(['-j', '0'])  # Use all CPU cores
     if not any(is_resource_use_flag(arg) for arg in regrtest_args):