XUnit.ITestOutputHelper.WriteLine not showing up

_From @fractionalJoe on June 15, 2018 22:19_

Issue Type: Bug

When running XUnit test with ITestOutputHelper to write output, the output is not available in the .Net Test Log window.
### Code
public class FunctionHandlerTest
{
#region fields
private readonly ITestOutputHelper Output;
#endregion

#region ctors
public FunctionHandlerTest(ITestOutputHelper output)
{
Output = output;
}
#endregion

#region methods
[Fact]
public void OutputTest()
{
Output.WriteLine(JsonConvert.SerializeObject(response, Formatting.Indented));
}
#endregion
}
### Output
#### (Edited slightly to replace names of customer' sensitive data with "....")

> Running test .......
>
> Microsoft (R) Build Engine version 15.7.179.6572 for .NET Core
> Copyright (C) Microsoft Corporation. All rights reserved.
>
> .....
>
> Build succeeded.
> 0 Warning(s)
> 0 Error(s)
>
> Time Elapsed 00:00:01.75
>
>
> [xUnit.net 00:00:00.3807450] Discovering: .....
> [xUnit.net 00:00:00.4582069] Discovered: .....
> [xUnit.net 00:00:00.4131600] Starting: .....
> [xUnit.net 00:00:00.5760216] Finished: .....
> .....: passed
>
> Total tests: 1. Passed: 1. Failed: 0. Skipped: 0

VS Code version: Code 1.24.1 (24f62626b222e9a8313213fb64b10d741a326288, 2018-06-13T17:47:35.732Z)
OS version: Linux x64 4.15.0-23-generic

System Info

|Item|Value|
|---|---|
|CPUs|Intel(R) Core(TM) i7-4710MQ CPU @ 2.50GHz (8 x 2493)|
|GPU Status|2d_canvas: unavailable_software
flash_3d: unavailable_software
flash_stage3d: unavailable_software
flash_stage3d_baseline: unavailable_software
gpu_compositing: unavailable_software
multiple_raster_threads: enabled_on
native_gpu_memory_buffers: disabled_software
rasterization: unavailable_software
video_decode: unavailable_software
video_encode: unavailable_software
vpx_decode: unavailable_software
webgl: unavailable_off
webgl2: unavailable_off|
|Load (avg)|1, 1, 1|
|Memory (System)|15.57GB (7.71GB free)|
|Process Argv|/usr/share/code/code --unity-launch|
|Screen Reader|no|
|VM|0%|

Extensions (3)

Extension|Author (truncated)|Version
---|---|---
dotnet-test-explorer|for|0.3.1
docomment|k--|0.1.0
csharp|ms-|1.15.2

_Copied from original issue: Microsoft/vscode#52079_