Fix test for unset `PPC_NUM_THREADS` environment variable #2913

Job	Run time
lint / python-lint	7s
lint / clang-format	13s
mac / macos-clang-build	2m 11s
mac / macos-clang-build-debug	2m 0s
ubuntu / ubuntu-clang-build (ubuntu-24.04)	1m 53s
ubuntu / ubuntu-clang-build (ubuntu-24.04-arm)	1m 53s
ubuntu / ubuntu-gcc-build-debug (ubuntu-24.04)	1m 33s
ubuntu / ubuntu-gcc-build (ubuntu-24.04-arm)	3m 25s
ubuntu / ubuntu-gcc-build (ubuntu-24.04)	1m 11s
windows / windows-msvc-build	2m 44s
windows / windows-msvc-build-debug	6m 9s
windows / windows-clang-build	2m 27s
ubuntu / ubuntu-clang-sanitizer-build (ubuntu-24.04)	1m 33s
ubuntu / ubuntu-clang-test (ubuntu-24.04)	2m 2s
ubuntu / ubuntu-clang-test (ubuntu-24.04-arm)	2m 15s
mac / macos-clang-test	3m 51s
windows / windows-clang-test	1m 21s
windows / windows-msvc-test	2m 14s
ubuntu / ubuntu-clang-sanitizer-test (ubuntu-24.04)	2m 3s
windows / windows-clang-test-extended	1m 16s
ubuntu / ubuntu-clang-test-extended	0s
ubuntu / ubuntu-gcc-test (ubuntu-24.04)	4m 32s
ubuntu / ubuntu-gcc-test (ubuntu-24.04-arm)	5m 21s
windows / windows-msvc-test-extended	1m 19s
ubuntu / ubuntu-clang-sanitizer-test-extended (ubuntu-24.04)	1m 48s
mac / macos-clang-test-extended	2m 17s
ubuntu / ubuntu-gcc-test-extended (ubuntu-24.04)	2m 38s
ubuntu / ubuntu-gcc-test-extended (ubuntu-24.04-arm)	3m 30s
ubuntu / ubuntu-gcc-build-codecov	0s
perf	0s
	1h 3m 46s

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