[flaky] vibe.core.net.1726 test

[MartinNowak]

Sometimes hangs indefinitely (v0.8.3-alpha.1).
https://ci.dlang.io/blue/organizations/jenkins/dlang-org%2Fphobos/detail/PR-5948/2/pipeline/#step-328-log-2266

[test.log](https://github.com/vibe-d/vibe-core/files/1710416/test.log)
[INFO] Running test vibe.core.net.1726
+ cd tests/vibe.core.net.1726
+ dub --compiler=dmd --override-config=vibe-d:core/libevent --build-mode=separate
Performing "debug" build using dmd for x86_64.
stdx-allocator 2.77.0: target for configuration "library" is up to date.
vibe-d:utils 0.8.3-alpha.1: target for configuration "library" is up to date.
vibe-d:data 0.8.3-alpha.1: target for configuration "library" is up to date.
vibe-d:core 0.8.3-alpha.1: target for configuration "libevent" is up to date.
tests ~master: building configuration "application"...
Linking...
To force a rebuild of up-to-date targets, run again with --force.
Running ./tests
core.exception.AssertError@source/app.d(58): Test has hung.
----------------
??:? _d_assert_msg [0x610b6a]
source/app.d:58 pure nothrow @nogc @safe void app.main().__lambda1() [0x555a50]
../../core/vibe/core/core.d:629 void vibe.core.core.makeTaskFuncInfo!(void delegate() @safe).makeTaskFuncInfo(ref void delegate() @safe).callDelegate(vibe.core.core.TaskFuncInfo*) [0x560f27]
../../core/vibe/core/core.d:1269 void vibe.core.core.CoreTask.run() [0x5655a6]
??:? void core.thread.Fiber.run() [0x6468a7]
??:? fiber_entryPoint [0x6465fa]
??:? [0xffffffff]
Program exited with code 1
script returned exit code 2

Here is the full test.log.

Also see dlang/ci#96