Pass in DbContextOptions to Microsoft.AspNet.Identity.EntityFramework.IdentityDbContext

[NeoAnomaly]

In the #418 discussed why this might be required.
I want a single context and for an unit tests purposes I need something like this:

var builder = new DbContextOptionsBuilder<SingleContextInMyApp>();
builder.UseInMemoryDatabase(persist: true);
var options = builder.Options;

using (var context = new SingleContextInMyApp(options))
{
...test code...
}

[HaoK]

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