VS2015 Tests no longer show in test explorer

I am currently developing in VS2015 (VS2017 not an option currently).

I have been successfully using VS2015, MSTest & test explorer for many months. Just recently the tests have stopped showing in test explorer.

In VS, the output window shows: 'test-mstest' returned '-2147450749'

I created a new project, and it also happens in that.

Global.json:

{
"projects": [ "src", "test" ],
"sdk": {
"version": "1.0.0-preview2-1-003177"
}
}

Project.json:

{
"version": "1.0.0-*",
"runtimes": { "win10-x64": {} },
"testRunner": "mstest",
"frameworks": {
"netcoreapp1.0": {
"dependencies": {},
"imports": [
"dnxcore50",
"portable-net45+win8"
]
}
},

"dependencies": {
"dotnet-test-mstest": "1.1.2-preview",
"MSTest.TestFramework": "1.1.18"
}
}

The following may or may not be relevant:

I am finding it difficult to properly determine which versions of sdk / netcoreapp etc I should be using together and with VS2015.

I have recently installed 3177 as it was recommended in a thread to fix the slow build times in VS2015 with earlier versions. Swapping back to 3131 does not seem to help.

Any help greatly appreciated.