improve AsyncIO robustness (#687)

Co-authored-by: Cédric Belmant <[email protected]>

Author: aviatesk

src/testing.jl

@@ -109,6 +109,7 @@ function Base.unsafe_read(io::AsyncIO, to::Ptr{UInt8}, nb::UInt)
             splice!(buffer, 1:n)
             written += n
         end
+        istaskdone(io.task) && throw(EOFError())
     end
 end