Cleanup: declare __all__ for Python scripts

Declare all to make public public API's explicit and
help detect unused code.

tests/utils/batch_load_blendfiles.py

@@ -2,6 +2,10 @@
 #
 # SPDX-License-Identifier: GPL-2.0-or-later
 
+__all__ = (
+    "main",
+)
+
 r"""
 Example usage:
 

tests/utils/bl_run_operators.py

@@ -7,6 +7,10 @@
 #
 # only error checked for here is a segfault.
 
+__all__ = (
+    "main",
+)
+
 import bpy
 import sys
 

tests/utils/bl_run_operators_event_simulate.py

@@ -82,7 +82,9 @@ or the context for executing the actions is not properly set (the case for timer
 
 This utility executes actions as if the user initiated them from a key shortcut.
 """
-
+__all__ = (
+    "main",
+)
 
 import os
 import sys

tests/utils/blender_headless.py

@@ -39,6 +39,10 @@ WAYLAND Environment Variables:
 
 Currently only WAYLAND is supported, other systems could be added.
 """
+__all__ = (
+    "main",
+)
+
 import subprocess
 import sys
 import signal