`UndefRefError` when trying to process `TestExt` on julia nightly

[lgoettgens]

We have been experiencing this for quite some time now in nemocas/AbstractAlgebra.jl.
See the below CI log. It always happens at the exact same place (the main file of our package extension on the Test stdlib), and only with julia nightly. I don't know if this is a problem with all pkg exts or just with Test, since this is the only pkg ext of this package.

@giordano pointed me here (original issue is julia-actions/julia-processcoverage#27)

https://github.com/Nemocas/AbstractAlgebra.jl/actions/runs/15162244026/job/42630901686#step:8:304

[...]
[ Info: CoverageTools.process_cov: processing src/utils.jl.2236.cov
[ Info: CoverageTools.process_cov: processing src/utils.jl.2243.cov
[ Info: CoverageTools.process_folder: Searching ext for .jl files...
[ Info: CoverageTools.process_folder: Searching ext/TestExt for .jl files...
[ Info: CoverageTools.process_file: Detecting coverage for ext/TestExt/Groups-conformance-tests.jl
[ Info: CoverageTools.process_cov: processing ext/TestExt/Groups-conformance-tests.jl.2174.cov
[ Info: CoverageTools.process_file: Detecting coverage for ext/TestExt/Mutating-ops.jl
[ Info: CoverageTools.process_cov: processing ext/TestExt/Mutating-ops.jl.2174.cov
[ Info: CoverageTools.process_file: Detecting coverage for ext/TestExt/Rings-conformance-tests.jl
[ Info: CoverageTools.process_cov: processing ext/TestExt/Rings-conformance-tests.jl.2174.cov
[ Info: CoverageTools.process_file: Detecting coverage for ext/TestExt/TestExt.jl
[ Info: CoverageTools.process_cov: processing ext/TestExt/TestExt.jl.2174.cov
ERROR: LoadError: UndefRefError: access to undefined reference
Stacktrace:
  [1] getindex
    @ ./essentials.jl:953 [inlined]
  [2] iterate
    @ ./array.jl:901 [inlined]
  [3] write
    @ ~/.julia/packages/CoverageTools/UhDjW/src/lcov.jl:44 [inlined]
  [4] #writefile##0
    @ ~/.julia/packages/CoverageTools/UhDjW/src/lcov.jl:27 [inlined]
  [5] open(::CoverageTools.LCOV.var"#writefile##0#writefile##1"{Vector{FileCoverage}}, ::String, ::Vararg{String}; kwargs::@Kwargs{})
    @ Base ./io.jl:410
  [6] open
    @ ./io.jl:407 [inlined]
  [7] writefile(outfile::String, fcs::Vector{FileCoverage})
    @ CoverageTools.LCOV ~/.julia/packages/CoverageTools/UhDjW/src/lcov.jl:26
  [8] top-level scope
    @ ~/work/_actions/julia-actions/julia-processcoverage/v1/main.jl:20
  [9] include(mod::Module, _path::String)
    @ Base ./Base.jl:[309](https://github.com/Nemocas/AbstractAlgebra.jl/actions/runs/15162244026/job/42630901686#step:8:311)
 [10] exec_options(opts::Base.JLOptions)
    @ Base ./client.jl:331
 [11] _start()
    @ Base ./client.jl:564
in expression starting at /home/runner/work/_actions/julia-actions/julia-processcoverage/v1/main.jl:20
Error: Process completed with exit code 1.

cc @fingolfin