Fix test for unset `PPC_NUM_THREADS` environment variable #2911

Job	Run time
lint / clang-format	7s
lint / python-lint	8s
mac / macos-clang-build-debug	2m 14s
ubuntu / ubuntu-gcc-build-debug (ubuntu-24.04)	1m 14s
ubuntu / ubuntu-clang-build (ubuntu-24.04-arm)	1m 41s
mac / macos-clang-build	2m 1s
ubuntu / ubuntu-gcc-build (ubuntu-24.04-arm)	3m 59s
ubuntu / ubuntu-gcc-build (ubuntu-24.04)	1m 24s
ubuntu / ubuntu-clang-build (ubuntu-24.04)	1m 9s
windows / windows-msvc-build-debug	5m 44s
windows / windows-msvc-build	2m 41s
windows / windows-clang-build	3m 9s
ubuntu / ubuntu-clang-test (ubuntu-24.04)	56s
ubuntu / ubuntu-clang-test (ubuntu-24.04-arm)	1m 18s
ubuntu / ubuntu-clang-sanitizer-build (ubuntu-24.04)	1m 38s
windows / windows-msvc-test	39s
mac / macos-clang-test	1m 16s
ubuntu / ubuntu-clang-test-extended	0s
windows / windows-msvc-test-extended	0s
windows / windows-clang-test	49s
ubuntu / ubuntu-clang-sanitizer-test (ubuntu-24.04)	1m 0s
ubuntu / ubuntu-gcc-test (ubuntu-24.04)	43s
ubuntu / ubuntu-gcc-test (ubuntu-24.04-arm)	57s
mac / macos-clang-test-extended	-1s
windows / windows-clang-test-extended	-1s
ubuntu / ubuntu-clang-sanitizer-test-extended	0s
ubuntu / ubuntu-gcc-test-extended	0s
ubuntu / ubuntu-gcc-build-codecov	0s
perf	0s
	34m 45s

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