Add ability to select individual tests in Tests tree

[Malivil]

Sometimes I want to run coverage against a handful of tests that are part of a larger test class. Currently AxoCover only lets you select individual tree nodes but it would be nice if you could CTRL+Click and/or SHIFT+Click to select groups or ranges.

[axodox]

I see no issue with adding this, however I am still working out some bugs with the support of NUnit and xUnit, and MSTestV2.