Fix parallel execution of directional plugin test ("Different tests were collected between gw0 and gw1")

[leroyvn]

This MR fixes a bug where the directional plugin test collection would fail when running pytest in parallel mode.

[Speierers]

I would be interested in understanding how this solves your issue?

But happy to merge this of course ;)

[leroyvn]

Indeed, sorry for not being more chatty about that! Parallel test collection was often failing because the xml_spectrum_keys list order would change between the different pytest threads. I'd get the following kind of error message:

🅒 mitsuba2 in mitsuba2 ➜ pytest -n 4 src/emitters/tests/test_directional.py
Running the full test suite. To skip slow tests, please run 'pytest -m 'not slow'
================================ test session starts ================================
platform darwin -- Python 3.7.7, pytest-5.4.1, py-1.8.1, pluggy-0.13.1
rootdir: /Users/vincent/Documents/src/thirdparty/mitsuba2, inifile: setup.cfg
plugins: xdist-1.30.0, forked-1.1.3
gw0 [18] / gw1 [18] / gw2 [18] / gw3 [18]
collecting 0 items / 1 error
====================================== ERRORS =======================================
_______________________________ ERROR collecting gw1 ________________________________
Different tests were collected between gw0 and gw1. The difference is:
--- gw0

+++ gw1

@@ -1,12 +1,12 @@

 src/emitters/tests/test_directional.py::test_construct
+src/emitters/tests/test_directional.py::test_eval[regular]
 src/emitters/tests/test_directional.py::test_eval[d65]
-src/emitters/tests/test_directional.py::test_eval[regular]
+src/emitters/tests/test_directional.py::test_sample_direction[direction0-regular]
 src/emitters/tests/test_directional.py::test_sample_direction[direction0-d65]
-src/emitters/tests/test_directional.py::test_sample_direction[direction0-regular]
+src/emitters/tests/test_directional.py::test_sample_direction[direction1-regular]
 src/emitters/tests/test_directional.py::test_sample_direction[direction1-d65]
-src/emitters/tests/test_directional.py::test_sample_direction[direction1-regular]
+src/emitters/tests/test_directional.py::test_sample_direction[direction2-regular]
 src/emitters/tests/test_directional.py::test_sample_direction[direction2-d65]
-src/emitters/tests/test_directional.py::test_sample_direction[direction2-regular]
 src/emitters/tests/test_directional.py::test_sample_ray[direction0-spatial_sample0]
 src/emitters/tests/test_directional.py::test_sample_ray[direction0-spatial_sample1]
 src/emitters/tests/test_directional.py::test_sample_ray[direction0-spatial_sample2]
============================== short test summary info ==============================
ERROR gw1
================================= 1 error in 0.71s ==================================

This kind of issue was mentioned here on the pytest-xdist repo.