Change the test projects to be the new csproj format

[kevbite]

The test projects still seem to be using the old style csproj formats, we should update these to the latest so we can have multitargeting tests for core and full framework

https://github.com/kevbite/CompaniesHouse.NET/blob/master/src/CompaniesHouse.Tests/CompaniesHouse.Tests.csproj https://github.com/kevbite/CompaniesHouse.NET/blob/master/src/CompaniesHouse.IntegrationTests/CompaniesHouse.IntegrationTests.csproj