Cleanup test project

[0xced]

• Update .NET Core target from .NET Core 1.0 to .NET Core 3.1 (LTS)
• Use latest versions of Microsoft.NET.Test.Sdk, xunit.runner.visualstudio and xunit
• Remove everything unneeded to build and run tests

Running tests now work in both IDEs and with dotnet test command line invocation.

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Oh, it's a duplicate of #40. But with the latest versions of the test dependencies... And it goes a little further by deleting unused file and even harmful files. The launchSettings.json file was causing JetBrains Rider to not be able to run the tests at all.

[nblumhardt]

Thanks for the PR! Would you mind if I merge #40 and we apply your additional changes on top? :-)

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That's fine.

[Numpsy]

I missed those uneeded netcoreapp1.0 conditions when I made my other PR, but I thought it useful to build the test project as both netcore 2.1 and 3.1 because the main Serilog project still seems to do both? (that's a call for $management$ though)

[Numpsy]

saying that, is it reasonable to run the tests against .net5 as well at this point?

[nblumhardt]

I think so 👍 - maybe we queue that up next.

RE coverage, the core Serilog lib is very conservative and tests on just about everything, but in sink/enricher projects we tend to keep it to one or two framework configurations in order to make things more manageable. I wouldn't be against adding some more targets, but the two here seem pretty reasonable :-)