runtime: TestCleanupLost failures

[gopherbot]

#!watchflakes
default <- pkg == "runtime" && test == "TestCleanupLost"

Issue created automatically to collect these failures.

Example (log):

=== RUN   TestCleanupLost
    mcleanup_test.go:336: expected 159999 cleanups to be executed, got 160000
--- FAIL: TestCleanupLost (0.15s)

— watchflakes

[gopherbot]

Found new dashboard test flakes for:

#!watchflakes
default <- pkg == "runtime" && test == "TestCleanupLost"

2025-12-03 18:29 go1.25-linux-amd64-longtest-noswissmap release-branch.go1.25@d5bfdcbc runtime.TestCleanupLost (log)

=== RUN   TestCleanupLost
    mcleanup_test.go:336: expected 159999 cleanups to be executed, got 160000
--- FAIL: TestCleanupLost (0.15s)

— watchflakes

[gopherbot]

Change https://go.dev/cl/731680 mentions this issue: runtime: correct a log message in TestCleanupLost