Fix test for unset `PPC_NUM_THREADS` environment variable #2909

Job	Run time
lint / python-lint	5s
lint / clang-format	8s
ubuntu / ubuntu-gcc-build (ubuntu-24.04-arm)	59s
ubuntu / ubuntu-gcc-build-debug (ubuntu-24.04)	52s
ubuntu / ubuntu-gcc-build (ubuntu-24.04)	55s
ubuntu / ubuntu-clang-build (ubuntu-24.04)	1m 1s
ubuntu / ubuntu-clang-build (ubuntu-24.04-arm)	53s
mac / macos-clang-build	59s
mac / macos-clang-build-debug	1m 4s
windows / windows-msvc-build	44s
windows / windows-clang-build	25s
windows / windows-msvc-build-debug	26s
windows / windows-clang-test-extended	1s
windows / windows-clang-test	1s
windows / windows-msvc-test	1s
windows / windows-msvc-test-extended	1s
mac / macos-clang-test-extended	1s
mac / macos-clang-test	1s
ubuntu / ubuntu-gcc-test	1s
ubuntu / ubuntu-gcc-test-extended	1s
ubuntu / ubuntu-clang-test	1s
ubuntu / ubuntu-clang-sanitizer-build	1s
ubuntu / ubuntu-gcc-build-codecov	1s
ubuntu / ubuntu-clang-sanitizer-test	1s
ubuntu / ubuntu-clang-test-extended	1s
ubuntu / ubuntu-clang-sanitizer-test-extended	1s
perf	0s
	8m 45s

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