runtime: TestLockOSThreadExit failures

[gopherbot]

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

Issue created automatically to collect these failures.

Example (log):

=== RUN   TestLockOSThreadExit
    proc_test.go:983: /home/swarming/.swarming/w/ir/x/t/go-build4249258931/testprog.exe LockOSThreadMain (3.989747ms): ok
    proc_test.go:989: /home/swarming/.swarming/w/ir/x/t/go-build4249258931/testprog.exe LockOSThreadAlt: exit status 2
    proc_test.go:991: want "OK\n", got "panic: runtime error: index out of range [2] with length 0\n\ngoroutine 1 [running]:\nmain.tidExists(0xba?)\n\t/home/swarming/.swarming/w/ir/x/w/goroot/src/runtime/testdata/testprog/syscalls_linux.go:25 +0xdd\nmain.LockOSThreadAlt()\n\t/home/swarming/.swarming/w/ir/x/w/goroot/src/runtime/testdata/testprog/lockosthread.go:93 +0x185\nmain.main()\n\t/home/swarming/.swarming/w/ir/x/w/goroot/src/runtime/testdata/testprog/main.go:34 +0x133\n"
--- FAIL: TestLockOSThreadExit (0.02s)

— watchflakes

[gopherbot]

Found new dashboard test flakes for:

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

2024-02-14 18:18 gotip-linux-amd64-newinliner go@b19164d5 runtime.TestLockOSThreadExit (log)

=== RUN   TestLockOSThreadExit
    proc_test.go:983: /home/swarming/.swarming/w/ir/x/t/go-build4249258931/testprog.exe LockOSThreadMain (3.989747ms): ok
    proc_test.go:989: /home/swarming/.swarming/w/ir/x/t/go-build4249258931/testprog.exe LockOSThreadAlt: exit status 2
    proc_test.go:991: want "OK\n", got "panic: runtime error: index out of range [2] with length 0\n\ngoroutine 1 [running]:\nmain.tidExists(0xba?)\n\t/home/swarming/.swarming/w/ir/x/w/goroot/src/runtime/testdata/testprog/syscalls_linux.go:25 +0xdd\nmain.LockOSThreadAlt()\n\t/home/swarming/.swarming/w/ir/x/w/goroot/src/runtime/testdata/testprog/lockosthread.go:93 +0x185\nmain.main()\n\t/home/swarming/.swarming/w/ir/x/w/goroot/src/runtime/testdata/testprog/main.go:34 +0x133\n"
--- FAIL: TestLockOSThreadExit (0.02s)

— watchflakes

[gopherbot]

Change https://go.dev/cl/567938 mentions this issue: runtime: be more defensive in tidExists about /proc/self/task/<tid>/stat