Add support for multiple project files in a directory

[amwaters]

.NET Test Explorer does not support projects in directories which contain more than one project file (e.g. csproj). It seems this is because dotnet commands are invoked by directory and not by project file.

1. Test discovery fails with MSB1001: Specify which project or solution file to use because this folder contains more than one project or solution file. (Limited workaround: add project filename to setting dotnet-test-explorer.testArguments)

2. Test execution fails because the directory build fails for the same reason. (The build step has no customisable arguments so there is no workaround for this.)

(.NET Core Test Explorer 0.7.7, .NET Core 6.0.201)