Add 'bare structure' of parallel_for task.

These currently do nothing but being boilerplate required to implement a new top level task.
4 jobs from parallel_for in 3 minutes 35 seconds (queued for 28 seconds)
Status Job ID Name Coverage
  Build
passed #2884
build_cmake

00:42

 
  Test
passed #2885
run_tests

00:42

 
  Sanitizer
passed #2887
run_address_sanitizer

01:14

passed #2886
run_thread_sanitizer

00:56