`TestIO#test_set_lineno_gets` returns wrong line number

[wks]

We have observed the following error twice:

1) Failure:
TestIO#test_set_lineno_gets [/home/runner/work/mmtk-ruby/mmtk-ruby/git/ruby/test/ruby/test_io.rb:1967]:
<1001> expected but was
<39>.

We only observed it on GitHub CI, but not locally. One of the PR runs that reproduced it is https://github.com/mmtk/mmtk-ruby/actions/runs/21025477165/job/60448844513?pr=149

Since it also manifests in (normal, release, Immix) but not StickyImmix, it may be related to #150

Here is an error log:

9_Build and test (normal, release, Immix).tar.gz