Run tests against multiple versions of ASP.Net Core

[jaredcnance]

We should be able to run tests against multiple versions of ASP.Net Core by defining the default versions in Directory.Build.props and then override by passing the versions as msbuild properties:

dotnet test ./test/UnitTests/UnitTests.csproj \
  /p:AspNetCoreVersion=2.0.0 \
  /p:EFCoreVersion=2.0.0 \
  /p:MicrosoftLoggingVersion=2.0.0

Steps to implementation:

1. Find a way to define the supported versions in a single place that can be imported by both build scripts
2. Import the supported versions
3. Iterate over the versions, passing them to the dotnet test command as shown above.

For now, the example projects will not work with Asp.Net Core v1, so as a POC, we should start with the following versions:

• 2.0.0
• 2.1.0

[jaredcnance]

Closing, see #302 for details.