dotnet: fix race condition in web test

          expect <<"EOF"

            set status 1

            spawn $env(src)/test

            proc abort { } { exit 2 }

            expect_before default abort

            expect -re {Now listening on: ([^\r]+)\r} {

              set url $expect_out(1,string)

              exit 1

            }

            send \x03

            expect_before timeout abort

            expect eof

            catch wait result

            exit [lindex $result 3]

          EOF