WIP/RFC: Allow workers to have a user-given number identifier

[nickrobinson251]

Now:

julia> runtests("test/testfiles/_happy_tests.jl", nworkers=2)
  Activating project at `~/repos/ReTestItems.jl`
[ Info: Scanning for test items in project `ReTestItems` at paths: test/testfiles/_happy_tests.jl
[ Info: Finished scanning for test items in 0.69 seconds. Scheduling 3 tests on pid 64940 with 2 worker processes and 2 threads per worker.
[ Info: Starting test workers
  Worker 1/65194:  [ Info: Starting test worker 1 on pid=65194, with 2 threads
  Worker 2/65193:  [ Info: Starting test worker 2 on pid=65193, with 2 threads
[ Info: Starting running test items
  Worker 1/65194:  14:26:02 | START (1/3) test item "happy 1" at test/testfiles/_happy_tests.jl:1
  Worker 2/65193:  14:26:02 | START (2/3) test item "happy 2" at test/testfiles/_happy_tests.jl:5
  Worker 1/65194:  14:26:02 | DONE  (1/3) test item "happy 1" <0.1 secs (91.2% compile), 17.24 K allocs (1.255 MB)
  Worker 2/65193:  14:26:02 | DONE  (2/3) test item "happy 2" <0.1 secs (91.2% compile), 17.24 K allocs (1.255 MB)
  Worker 2/65193:  14:26:03 | START (3/3) test item "happy 3" at test/testfiles/_happy_tests.jl:9
[ Info: All tests on worker 1 completed. Closing Worker(num=1, pid=65194).
  Worker 2/65193:  14:26:03 | DONE  (3/3) test item "happy 3" <0.1 secs (<0.1% compile), 1.62 K allocs (116.776 KB)
[ Info: All tests on worker 2 completed. Closing Worker(num=2, pid=65193).
[ Tests Completed: 3/3 test items were run.
Test Summary:                        | Pass  Total  Time
ReTestItems                          |    3      3  3.2s
  test                               |    3      3
    test/testfiles                   |    3      3
      test/testfiles/_happy_tests.jl |    3      3
        happy 1                      |    1      1  0.0s
        happy 2                      |    1      1  0.0s
        happy 3                      |    1      1  0.0s