Add support for multiple project files in a directory

.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)