Fix IOBuffer data race in stdio_loop / take!

[DaniGlez]

I got some ParallelTestRunner 2.5.0 failures that seem to be related to concurrent accesses to an IOBuffer; I will post a full stack trace in comments. This PR is AI-assisted, but it seems reasonable to the best of my knowledge and I have manually tested and verified that it does indeed fix the issue.

Detailed description:
Two background tasks (stdout + stderr) wrote to the same IOBuffer concurrently, and the main test loop called take! without any synchronisation. Julia's IOBuffer is not thread-safe, so this caused torn reads of io.size vs io.data under higher I/O load, manifesting as:

DimensionMismatch: Attempted to wrap a MemoryRef of length N with
an Array of size dims=(M,)

The proposed fix involves adding a ReentrantLock to PTRWorker; both stdio_loop write tasks and the take! call in runtests hold the lock.

[DaniGlez]

Example stack trace:

Caught an error, stopping...
ERROR: LoadError: TaskFailedException
Stacktrace:
 [1] wait(t::Task)
   @ Base ./task.jl:370
 [2] runtests(mod::Module, args::ParallelTestRunner.ParsedArgs; testsuite::Dict{String, Expr}, init_code::Expr, init_worker_code::Expr, test_worker::Returns{Nothing}, stdout::IOContext{Base.PipeEndpoint}, stderr::IOContext{Base.PipeEndpoint}, max_worker_rss::Int64)
   @ ParallelTestRunner [redacted]/packages/ParallelTestRunner/EzCGZ/src/ParallelTestRunner.jl:1138
 [3] #runtests#58
   @ [redacted]/packages/ParallelTestRunner/EzCGZ/src/ParallelTestRunner.jl:1282 [inlined]
 [4] top-level scope
   @ [redacted]/test/runtests.jl:71
 [5] include(fname::String)
   @ Main ./sysimg.jl:38
 [6] top-level scope
   @ none:6

    nested task error: DimensionMismatch: Attempted to wrap a MemoryRef of length 30 with an Array of size dims=(507,) which is invalid because prod(dims) = 507 > 30 so that the array would have more elements than the underlying memory can store.
    Stacktrace:
     [1] invalid_wrap_err(len::Int64, dims::Tuple{Int64}, proddims::Int64)
       @ Base ./array.jl:3102
     [2] _wrap
       @ ./array.jl:3094 [inlined]
     [3] wrap
       @ ./array.jl:3120 [inlined]
     [4] take!(io::IOBuffer)
       @ Base ./iobuffer.jl:469
     [5] (::ParallelTestRunner.var"#33#49"{Dict{String, Expr}, Expr, Expr, Returns{Nothing}, Int64, ParallelTestRunner.ParsedArgs, Channel{Tuple}, ParallelTestRunner.TestIOContext, ParallelTestRunner.var"#stop_work#41"{Vector{Task}}, ReentrantLock, Dict{String, Float64}, Vector{Any}, Vector{String}})()
       @ ParallelTestRunner [redacted]/packages/ParallelTestRunner/EzCGZ/src/ParallelTestRunner.jl:1064
in expression starting at [redacted]/test/runtests.jl:14
ERROR: Package ... errored during testing
Stacktrace:
 [1] pkgerror(msg::String)
   @ Pkg.Types /usr/local/julia/share/julia/stdlib/v1.11/Pkg/src/Types.jl:68
 [2] test(ctx::Pkg.Types.Context, pkgs::Vector{Pkg.Types.PackageSpec}; coverage::Bool, julia_args::Cmd, test_args::Cmd, test_fn::Nothing, force_latest_compatible_version::Bool, allow_earlier_backwards_compatible_versions::Bool, allow_reresolve::Bool)
   @ Pkg.Operations /usr/local/julia/share/julia/stdlib/v1.11/Pkg/src/Operations.jl:2128
 [3] test
   @ /usr/local/julia/share/julia/stdlib/v1.11/Pkg/src/Operations.jl:2011 [inlined]
 [4] test(ctx::Pkg.Types.Context, pkgs::Vector{Pkg.Types.PackageSpec}; coverage::Bool, test_fn::Nothing, julia_args::Vector{String}, test_args::Cmd, force_latest_compatible_version::Bool, allow_earlier_backwards_compatible_versions::Bool, allow_reresolve::Bool, kwargs::@Kwargs{io::IOContext{IO}})
   @ Pkg.API /usr/local/julia/share/julia/stdlib/v1.11/Pkg/src/API.jl:481
 [5] test(pkgs::Vector{Pkg.Types.PackageSpec}; io::IOContext{IO}, kwargs::@Kwargs{coverage::Bool, julia_args::Vector{String}, force_latest_compatible_version::Bool, allow_reresolve::Bool})
   @ Pkg.API /usr/local/julia/share/julia/stdlib/v1.11/Pkg/src/API.jl:159
 [6] test(; name::Nothing, uuid::Nothing, version::Nothing, url::Nothing, rev::Nothing, path::Nothing, mode::Pkg.Types.PackageMode, subdir::Nothing, kwargs::@Kwargs{coverage::Bool, julia_args::Vector{String}, force_latest_compatible_version::Bool, allow_reresolve::Bool})
   @ Pkg.API /usr/local/julia/share/julia/stdlib/v1.11/Pkg/src/API.jl:174
 [7] top-level scope
   @ none:7

[giordano]

This is now live in v2.5.1

[DaniGlez]

Thanks!