Unit tests no longer discovered in VS 15.6.3 (19 Mar 2018)

Myself and another colleague have recently upgraded to VS 15.6.3:
https://docs.microsoft.com/en-us/visualstudio/releasenotes/vs2017-relnotes#15.6.3

Since upgrading, Resharper no longer discovers any of the tests in our .NET Core solution. Unit test explorer is empty, and the quick-run icons in the left margin of the code editor are not present.

The nUnit test adaptor is working fine, as the dotnet command line tools are able to locate the tests without issue.

I must say, we are not having a very good experience with Resharper + .NET Core so far. It's very slow, and the experience is very buggy. I don't know how much of it is Visual Studio, and how much of it is Resharper, but my team are getting increasingly frustrated with this toolset.