test: fix flaky wpt/test-timers

Change the wpt tests to not run in parallel in our Python test runner.
On my local machine, this changes the time needed to run the suite from
about 2 seconds to about 5 seconds, but it makes the test suite much
more reliable.

Fixes: nodejs#37672

test/wpt/testcfg.py

@@ -3,4 +3,4 @@
 import testpy
 
 def GetConfiguration(context, root):
-  return testpy.ParallelTestConfiguration(context, root, 'wpt')
+  return testpy.SimpleTestConfiguration(context, root, 'wpt')