Increase parallelism on Linux WPT testing

The time taken by each chunk is uneven, with WPT-1 the longest
before this change at 30~45 minutes. This reduces it to 15~20 minutes.

Surprisingly, increasing the number of processes seems to also make
`test_element_in_collection` in `/webdriver/tests/execute_script/cyclic.py`
**unexpectedly pass**. This happened reliably in three different runs:

https://community-tc.services.mozilla.com/tasks/S9O27WJvSa6j2PSjcRcbBA/runs/2

tests/wpt/metadata/webdriver/tests/execute_script/cyclic.py.ini

@@ -3,4 +3,5 @@
     expected: FAIL
 
   [test_element_in_collection]
-    expected: FAIL
+    expected:
+      if os == "mac": FAIL