DotNet commands do not respect the verbosity

[Roemer]

As the title says, the DotNetCommands do not respect the verbosity setting. The problem seems to be that the verbosity expects a lowercase text (see https://docs.microsoft.com/en-us/dotnet/core/tools/dotnet?tabs=netcore21) but cake just uses the Enum.ToString().
I will create a PR which adds a ToLower().

[Roemer]

Can be verified like this:

var settings = new DotNetCoreTestSettings
{
    Verbosity = DotNetCoreVerbosity.Normal,
};
// This won't print out the tests that are executed
DotNetCoreTest("some.dll", settings);

var settings = new DotNetCoreTestSettings
{
    ArgumentCustomization = args=>args.Append("-v=normal")
};
// This will print out the tests that are executed
DotNetCoreTest("some.dll", settings);