Make output assertion deterministic for test for #7172

[iffy]

Test t7172.nim sometimes fails because of race conditions in threading and the expected output. This change makes it always pass while still preserving (as best I can tell) the functionality of the test.

[iffy]

Here's what a failure looks like:

Category: threads
Name: tests/threads/t7172.nim c  --threads:on -d:release
Action: run
Result: reOutputsDiffer
-------- Expected -------
In doStuff()
In initProcess()
initProcess() done
TEST
Crashes before getting here!

--------- Given  --------
In doStuff()
In initProcess()
TEST
initProcess() done
Crashes before getting here!

-------------------------

[github-actions]

Thanks for your hard work on this PR!
The lines below are statistics of the Nim compiler built from 964afd3

Hint: mm: orc; opt: speed; options: -d:release
162676 lines; 9.870s; 665.039MiB peakmem