killNode(): Check failures on R-universe Linux

https://henrikbengtsson.r-universe.dev/parallelly

Unit test:

https://github.com/futureverse/parallelly/blob/5e77854ff838a5f1a0e567896a15b19453b31058/inst/testme/test-killNode.R#L70-L86

fails on R-universe Linux builders (only):

```
2025-05-10T03:33:43.9582552Z Running ‘test-killNode.R’ [1s/31s]
2025-05-10T03:33:43.9648487Z [9s/35s] ERROR
2025-05-10T03:33:43.9656251Z Running the tests in ‘tests/test-killNode.R’ failed.
2025-05-10T03:33:43.9657591Z Last 13 lines of output:
2025-05-10T03:33:43.9658926Z TRUE TRUE
2025-05-10T03:33:43.9659254Z Node 1 Node 2
2025-05-10T03:33:43.9660012Z TRUE TRUE
2025-05-10T03:33:43.9660353Z Node 1 Node 2
2025-05-10T03:33:43.9660622Z TRUE TRUE
2025-05-10T03:33:43.9660885Z Node 1 Node 2
2025-05-10T03:33:43.9661146Z TRUE TRUE
2025-05-10T03:33:43.9661415Z Node 1 Node 2
2025-05-10T03:33:43.9661676Z TRUE TRUE
2025-05-10T03:33:43.9661947Z Node 1 Node 2
2025-05-10T03:33:43.9662215Z TRUE TRUE
2025-05-10T03:33:43.9662517Z Error in eval(ei, envir) :
2025-05-10T03:33:43.9662995Z One or more cluster nodes are still running after 30 seconds
2025-05-10T03:33:43.9663701Z Calls: ... eval -> eval -> source -> withVisible -> eval -> eval
2025-05-10T03:33:43.9664296Z Execution halted
2025-05-10T03:33:44.2620175Z * checking for unstated dependencies in vignettes ... OK
2025-05-10T03:33:44.2783828Z * checking package vignettes ... OK
2025-05-10T03:33:44.2786236Z * checking running R code from vignettes ... SKIPPED
2025-05-10T03:33:44.2790205Z * checking re-building of vignette outputs ... SKIPPED
2025-05-10T03:33:44.2793032Z * DONE
2025-05-10T03:33:44.2796384Z
2025-05-10T03:33:44.2797794Z Status: 1 ERROR
2025-05-10T03:33:44.2801122Z See
2025-05-10T03:33:44.2801860Z ‘/__w/cran/cran/parallelly.Rcheck/00check.log’
2025-05-10T03:33:44.2802300Z for details.
2025-05-10T03:33:44.2803860Z
2025-05-10T03:33:44.4912519Z ##[error]Check: tests, Result: ERROR
Running ‘test-all.equal.connection.R’
Running ‘test-as.cluster.R’
Running ‘test-availableCores.R’
Running ‘test-availableWorkers.R’
Running ‘test-cgroups.R’
Running ‘test-cpuLoad.R’
Running ‘test-freeCores.R’
Running ‘test-freePort.R’
Running ‘test-isConnectionValid.R’
Running ‘test-isForkedChild.R’
Running ‘test-killNode.R’ [1s/31s]
Running the tests in ‘tests/test-killNode.R’ failed.
Last 13 lines of output:
TRUE TRUE
Node 1 Node 2
TRUE TRUE
Node 1 Node 2
TRUE TRUE
Node 1 Node 2
TRUE TRUE
Node 1 Node 2
TRUE TRUE
Node 1 Node 2
TRUE TRUE
Error in eval(ei, envir) :
One or more cluster nodes are still running after 30 seconds
Calls: ... eval -> eval -> source -> withVisible -> eval -> eval
Execution halted
```

# See also

Full logs from 2025-05-09: [logs_38365589033.zip](https://github.com/user-attachments/files/20371764/logs_38365589033.zip)

Start with inst/testme/test-killNode.R lines 70-86 and compare its behavior with the linked R-universe Linux logs. Run tests/test-killNode.R in a comparable Linux environment and identify why cluster nodes remain running after 30 seconds. Done means the test completes successfully on the affected Linux builders.

Written by the indexing model from the issue text.