[llvm] [LIT] Workaround the 60 processed limit on Windows (PR #157759)

[LLVM Continuous Integration via llvm-commits]

llvm-ci wrote:

LLVM Buildbot has detected a new failure on builder `lld-x86_64-ubuntu-fast` running on `as-builder-4` while building `llvm` at step 6 "test-build-unified-tree-check-all".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/33/builds/27268

<details>
<summary>Here is the relevant piece of the build log for the reference</summary>

```
Step 6 (test-build-unified-tree-check-all) failure: test (failure)
******************** TEST 'lit :: windows-pools.py' FAILED ********************
Exit Code: 127

Command Output (stdout):
--
# RUN: at line 3
rm -Rf /home/buildbot/worker/as-builder-4/ramdisk/lld-x86_64/build/utils/lit/tests/Output/windows-pools.py.tmp.dir && mkdir -p /home/buildbot/worker/as-builder-4/ramdisk/lld-x86_64/build/utils/lit/tests/Output/windows-pools.py.tmp.dir
# executed command: rm -Rf /home/buildbot/worker/as-builder-4/ramdisk/lld-x86_64/build/utils/lit/tests/Output/windows-pools.py.tmp.dir
# executed command: mkdir -p /home/buildbot/worker/as-builder-4/ramdisk/lld-x86_64/build/utils/lit/tests/Output/windows-pools.py.tmp.dir
# RUN: at line 4
python -c "for i in range(20): open(rf'/home/buildbot/worker/as-builder-4/ramdisk/lld-x86_64/build/utils/lit/tests/Output/windows-pools.py.tmp.dir/file{i}.txt', 'w').write('RUN:')"
# executed command: python -c 'for i in range(20): open(rf'"'"'/home/buildbot/worker/as-builder-4/ramdisk/lld-x86_64/build/utils/lit/tests/Output/windows-pools.py.tmp.dir/file{i}.txt'"'"', '"'"'w'"'"').write('"'"'RUN:'"'"')'
# .---command stderr------------
# | 'python': command not found
# `-----------------------------
# error: command failed with exit status: 127

--

********************


```

</details>

https://github.com/llvm/llvm-project/pull/157759