Merge benchmarks back into main repo

[rowanmiller]

Resolves #7625

In a non-perf run (i.e. when you just run locally, and on CI) the tests take 68sec to execute on my laptop - so probably under a minute on most hardware.

@bricelam could you take a quick look over this one. It is mostly just a copy of the code except I changed assembly/project names (and namespaces) around.

• Benchmarks.Core (shared infra) => Microsoft.EntityFrameworkCore.Benchmarks
• Benchmarks (EF Core benchmarks) => Microsoft.EntityFrameworkCore.Benchmarks.EFCore
• Benchmarks.EF6 (EF6 benchmarks) => Microsoft.EntityFrameworkCore.Benchmarks.EF6

I'll also add a comment to the other piece of code I changed.

[rowanmiller]

This used to use Microsoft.Extensions.Internal.RuntimeEnvironment.RuntimeType but that now returns CoreCLR even when running on the net452 output in the xunit console runner. What I have now is what we used to have before we split the benchmarks out to a separate repo (where the compilation of tests may not have matched the compilation of product).

[smitpatel]

Do we need attribute on each method if the class is already marked with it?

[rowanmiller]

Possibly not, I'll play around with that after I've got the code merged. I wanted to move it over with minimal changes to start with 😄

[ajcvickers]

I'm happy with getting this merged in first and then making additional changes as necessary.

[ErikEJ]

This is great, looking forward to try them out with the SQLCE provider

[bricelam]

Might be able to get away with just <TargetFramework> (no s) here if the EF and ASP.NET targets can handle that.

[bricelam]

Might also need to exclude this from non-Windows. @natemcmaster probably knows how to do that...

[natemcmaster]

Example: https://github.com/aspnet/EntityFramework.Tools/blob/dev/build/repo.props

[bricelam]

LGTM (just looked at *.csproj files)