Fix test for unset `PPC_NUM_THREADS` environment variable #2903

Job	Run time
lint / clang-format	12s
lint / python-lint	8s
ubuntu / ubuntu-gcc-build-debug (ubuntu-24.04)	36s
ubuntu / ubuntu-gcc-build (ubuntu-24.04)	1m 14s
ubuntu / ubuntu-gcc-build (ubuntu-24.04-arm)	1m 14s
mac / macos-clang-build	1m 19s
mac / macos-clang-build-debug	1s
ubuntu / ubuntu-clang-build (ubuntu-24.04-arm)	1s
windows / windows-msvc-build	1s
windows / windows-msvc-build-debug	1s
windows / windows-clang-build	9s
ubuntu / ubuntu-clang-build (ubuntu-24.04)	1s
ubuntu / ubuntu-clang-sanitizer-build	1s
ubuntu / ubuntu-gcc-test	1s
windows / windows-msvc-test	1s
ubuntu / ubuntu-clang-test	1s
windows / windows-msvc-test-extended	1s
ubuntu / ubuntu-clang-sanitizer-test	1s
ubuntu / ubuntu-clang-sanitizer-test-extended	1s
ubuntu / ubuntu-gcc-test-extended	1s
ubuntu / ubuntu-clang-test-extended	1s
ubuntu / ubuntu-gcc-build-codecov	1s
windows / windows-clang-test-extended	1s
windows / windows-clang-test	1s
mac / macos-clang-test	1s
mac / macos-clang-test-extended	1s
perf	-1s
	5m 10s

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