Upgrade tests to use Xunit 2.0
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.
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.
In Sys..Xml.XPath.XmlDoc..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.
In System.Xml.XPath 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.
Update reference to build tools to 1.0.16-prerelease
Disable running TPL tests by default.

On .NET Core, the TPL tests are hanging and need investigation.
Disable them by default on the command line build for now.
Fix merge error in MetadataReader test project