Test Cleanup & CI Fixes

[Turnerj]

Started with organising the tests to match the folder structure of the main library. Continued to have the tests inherit from the same base (allowing easier cleanup per test - particularly around cleaning up the MongoDB driver) however will also include fixing up some neglected test cases.

This also helps get the tests in a better shape for another refactor/rename of core components in the main library.

[codecov]

Codecov Report

Merging #28 into master will decrease coverage by 0.05%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #28      +/-   ##
==========================================
- Coverage   93.43%   93.38%   -0.06%     
==========================================
  Files          53       54       +1     
  Lines        1736     1753      +17     
==========================================
+ Hits         1622     1637      +15     
- Misses        114      116       +2

Impacted Files	Coverage Δ
MongoFramework/MongoDbContextOptions.cs	100% <100%> (ø)
MongoFramework/Infrastructure/MongoDbUtility.cs	100% <100%> (ø)	⬆️
MongoFramework/MongoDbContext.cs	71.83% <100%> (+2.13%)	⬆️
...ture/EntityRelationships/EntityMapperExtensions.cs	96.87% <0%> (-3.13%)	⬇️

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[Turnerj]

Test project has been migrated to new .csproj format and now builds both .Net 4.6.1 and .Net Core 2.0 for testing.

Due to a difference with how .Net Core supports configuration, there is a slight decrease in functionality/usage in .Net Core around creating contexts via connection string name. There is a new constructor that allows passing in an "options" object that allows configuring the same type of data (connection string and database). This "options" object follows similar functionality in EF Core.