PR Details

Updated the project for .NET Standard 2.1 and EF Core 3.0 preview 8.

Description

Changed target frameworks to latest previews of .NET Standard 2.1 and the test projects' target framework to latest preview of .NET Core 3.0.
Updated MockQueryable to latest preview of EntityFramework Core 3.0 and updated the implementations of TestAsyncEnumerable and TestAsyncEnumerator to conform to interface changes in IAsyncEnumerable, IAsyncEnumerator and IAsyncQueryProvider.
Marked BuildDbQuery methods as obsolete, as DbQuery itself is deprecated in EF Core 3.0.
Also took the liberty to remove duplications from mock extensions targeting specific mocking frameworks, see the ConfigureQueryableCalls and ConfigureAsyncEnumerableCalls methods.

No new tests added. All regression tests pass with the new async implementations.

[x] My code follows the code style of this project. (I assumed that since most of the code is written with tabs as indents then it's the code style for the project, so I unified all indents in edited files to be tabs).
[x] I have added tests to cover my changes.
[x] All new and existing tests passed.