Upgrade tests to use Xunit 2.0
Fix DEBUG constant name
Fix some more XML docs
Prepare metadata reader tests to be portable
Build metadata reader tests as PCL
Merge branch 'xunit2' into test_on_coreclr

Conflicts:
	src/System.Numerics.Vectors/tests/Matrix3x2Tests.cs
	src/System.Numerics.Vectors/tests/Matrix4x4Tests.cs
	src/System.Numerics.Vectors/tests/PlaneTests.cs
	src/System.Numerics.Vectors/tests/QuaternionTests.cs
	src/System.Numerics.Vectors/tests/Vector2Tests.cs
	src/System.Numerics.Vectors/tests/Vector4Tests.cs
	src/System.Reflection.Metadata/tests/TestUtilities/WorkItemAttribute.cs
Merge branch 'mdreader-portable-tests' into test_on_coreclr

Conflicts:
	src/System.Reflection.Metadata/tests/Metadata/MetadataReaderTests.cs
	src/System.Reflection.Metadata/tests/System.Reflection.Metadata.Tests.csproj
	src/System.Reflection.Metadata/tests/packages.config
Update XML tests to register a code page provider

Required for the tests to pass on .NET Core.
Improve ActionBlock.Post perf for SingleProducerConstrained
Make SpscTargetCore.OfferMessage use Post method on fast path
Remove TargetCore.Post method as it doesn't add to perf.
Disable 4 PLINQ tests against GitHub issue corefx/240
Disable MetadataReader test against GitHub issue corefx/260
Disable 4 MetadataReader tests against GitHub issue corefx/264
Convert TPL tests to a portable library compiled against contracts.
Stop Micr..Win32.Registry from copying contract assemblies to output
Stop SIMD tests from copying contract assemblies to output
Include referenced fx libraries in packages.config for SIMD tests
Stop copying contract assemblies to output folder for MDReader tests

And add a reference to System.Threading.Tasks.Parallel 4.0.0
In Sys..Xml.XPath.XDoc..Tests, add ref to fx libs and fix pkg refs

Add references to System.Text.Encoding 4.0.10 and
System.Text.Encoding.CodePages 4.0.0, as well as make packages.config
consistent with framework references in the project.