runtime/pprof: new test TestLabelSystemstack is flaky #50007
Description
greplogs --dashboard -md -l -e 'TestLabelSystemstack':
2021-12-05T12:50:44-ecf6b52/freebsd-arm-paulzhol
2021-12-05T12:50:44-ecf6b52/linux-mipsle-rtrk
2021-12-04T17:16:07-3396878/freebsd-arm-paulzhol
2021-12-04T17:16:07-3396878/openbsd-mips64-jsing
2021-12-04T11:59:35-1876b38/freebsd-arm-paulzhol
2021-12-04T11:59:35-1876b38/openbsd-mips64-jsing
2021-12-04T04:50:55-549cfef/freebsd-arm-paulzhol
2021-12-04T01:07:21-821bf04/linux-386
2021-12-03T21:25:05-d20a0bf/linux-ppc64le-buildlet
2021-12-03T21:25:05-d20a0bf/openbsd-mips64-jsing
2021-12-03T16:11:38-9b0de08/linux-riscv64-unmatched
Test originally added in https://golang.org/cl/351751 for #48577 and reverted in https://golang.org/cl/360757 because it was extremely flaky. Re-added https://golang.org/cl/367200. It is much less flaky now, but still a bit flaky.