Python: fix failing tests when building bpy module

* Use Python executable from lib folder since it's not installed.
* Make bpy module test work for portable install.
* Disable gtests which don't work with different Python link flags
  and shared library locations.

Ref D15957

tests/blender_as_python_module/import_bpy.py

@@ -1,4 +1,8 @@
 # SPDX-License-Identifier: GPL-2.0-or-later
 
+# Add directory with module to the path.
+import sys
+sys.path.append(sys.argv[1])
+
 # Just import bpy and see if there are any dynamic loader errors.
 import bpy