libsubprocess/test: Fix racy fd count in test #10105

Job	Run time
validate commits	12s
Generate build matrix	7s
python linting	33s
spelling	6s
bookworm - test-install	12m 4s
bookworm - 32 bit	13m 16s
noble - test-install	12m 41s
el9 - test-install	12m 15s
alpine - test-install	12m 59s
fedora40 - test-install	12m 11s
jammy - test-install	11m 55s
focal - py3.8	16m 38s
bookworm - gcc-12,content-s3,distcheck	19m 55s
fedora40 - clang-18	16m 43s
coverage	23m 45s
el8 - ascii	19m 14s
el8 - test-install	13m 22s
el8 - system,coverage	7m 22s
inception	21m 27s
flux-pmix check	12m 54s
flux-sched check	6m 56s
flux-accounting check	4m 40s
macos build only	3m 38s
flux-pam check	4m 14s
Generate docker manifest	1s
	4h 19m 8s

Provide feedback