It takes a long time to run the full C# driver test suite. Part of the reason is that the test suite is run 3 times, once for each target framework.

Add new targets to build.cake to support running tests against just one target framework.

.\build.ps -target testnet452
.\build.ps -target testnetstandard15
.\build.ps -target testnetstandard20

Note: the target frameworks are the ones the driver targets. The test projects actually target a suitable version of .NET Core to run the tests for the netstandard versions of the driver.

This will be useful during development to run the full test suite in 1/3 the time by selecting which target framework to test.