Add support for WebApplicationFactory

[OskarKlintrot]

Fixes #435

No test cases for WebApplicationFactory yet, I open this PR just because it's much easier to show the diff in a PR.

[dnfadmin]

All CLA requirements met.

[OskarKlintrot]

@SimonCropp I accidently check the wrong radio button in the CLA, can you revoke my signing so I can redo it? I can't change it now unfortunately.

[SimonCropp]

if u run two tests in parallel, wont the results overlap?

[OskarKlintrot]

You're right, they will mess with each others data :(

[OskarKlintrot]

Now the tests can be run in parallel. It tok a lot of code and some new public facing api's though...

[OskarKlintrot]

I totally forgot that the interceptor is singleton in each WebApplicationFactory, they aren't sharing the same singleton instance (which I think was the case in EF6, didn't you put DbInterception in Application_Start or something similar?). Now the PR looks a bit cleaner.

[OskarKlintrot]

I haven't written any docs btw, I'll do it if you think this PR makes sense :)

[SimonCropp]

i think this looks good. if you add some docs we can merge and deploy

[OskarKlintrot]

Great! I added a section about WebApplicationFactory, I hope it makes sense! I changed a few variables in the tests to make the docs coherent.