runtime/pprof: TestCPUProfileMultithreadMagnitude failing frequently since 2021-09-27

[bcmills]

greplogs --dashboard -md -l -e 'FAIL: TestCPUProfileMultithreadMagnitude'

2021-10-19T07:45:46-ee92daa/linux-386-longtest
2021-10-18T21:21:54-6c0daa7/linux-386-longtest
2021-10-18T19:46:27-394a1ad/linux-amd64-longtest
2021-10-15T20:29:48-85cbdda/linux-riscv64-jsing
2021-10-12T20:20:41-d032b2b/linux-amd64-wsl
2021-10-11T12:53:11-34f7b1f/linux-riscv64-unmatched
2021-10-08T17:32:31-d7ba1d2/linux-riscv64-unmatched
2021-10-08T14:08:12-5b9206f/linux-riscv64-unmatched
2021-10-06T16:11:20-aecf4b1/linux-riscv64-unmatched
2021-10-05T16:06:49-060cd73/linux-riscv64-unmatched
2021-10-02T13:52:59-07a7c6f/linux-riscv64-unmatched
2021-10-01T17:27:29-3357624/linux-riscv64-unmatched
2021-10-01T17:27:24-952df98/linux-riscv64-jsing
2021-10-01T04:30:22-822f349/linux-amd64-wsl
2021-09-30T16:43:32-9112d29/linux-riscv64-unmatched
2021-09-27T18:58:36-5b90958/linux-amd64-longtest
2021-09-27T18:58:36-5b90958/linux-amd64-wsl
2021-09-27T18:58:29-8cfd8c3/linux-amd64-longtest
2021-09-27T18:58:29-8cfd8c3/linux-amd64-wsl

[bcmills]

@rhysh, @prattmic: could these failures be related to the per-thread changes that landed around Sep. 27?

[prattmic]

Duplicate of #49065