Move to xunit.console for CoreClr tests

[jaredpar]

Previously we were using xunit.console for desktop tests and dotnet-xunit for our
CoreClr tests. This change unifies us on top of xunit.console (now that it has a
netcoreapp2.0 version available).

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Customer scenario

What does the customer do to get into this situation, and why do we think this
is common enough to address for this release. (Granted, sometimes this will be
obvious "Open project, VS crashes" but in general, I need to understand how
common a scenario is)

Bugs this fixes

(either VSO or GitHub links)

Workarounds, if any

Also, why we think they are insufficient for RC vs. RC2, RC3, or RTW

Risk

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Performance impact

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Is this a regression from a previous update?

Root cause analysis

How did we miss it? What tests are we adding to guard against it in the future?

How was the bug found?

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Test documentation updated?

If this is a new non-compiler feature or a significant improvement to an existing feature, update https://github.com/dotnet/roslyn/wiki/Manual-Testing once you know which release it is targeting.

[jaredpar]

In changing the coreclr tests to run on a new runner i broke all of the desktop tests. Impressive.