libgit2: Testing managed and unmanaged transport

[pjbgf]

Given that managed.InitManagedTransport is an unreversible operation, and that we want to support both unmanaged and managed transport for at least a minor release, we might want to run tests against both when that makes sense, and skip when a specific test is transport sensitive.

Something around the lines of:

func TestMain(m *testing.M) {
	// execute test against unmanaged transport
	code := m.Run()
	if code != 0 {
		os.Exit(code)
	}
	managed.InitManagedTransport(logr.Discard())

	// execute test against managed transport
	code = m.Run()
	os.Exit(code)
}

func TestManaged_something(t *testing.T) {
	if !managed.Enabled() {
		t.Skip()
	}

	// Managed only tests...
}

func TestUnmanaged_something(t *testing.T) {
	if managed.Enabled() {
		t.Skip()
	}

	// Unmanaged only tests...
}

func TestBoth_something(t *testing.T) {
	// Test supported for both managed and unmanaged...
}

Potential follow-up of #744, #727 and #718.

[aryan9600]

#746 addresses the same issue in a different way. Instead of using a TestMain and skipping tests, when required it places all the tests in different files strategically, leveraging the fact that golang runs tests in a package based on an alphabetical order of the file names in the package. This lets us run all the tests using unmanaged transports prior to any other tests, letting us test both managed and unmanaged transports.

[pjbgf]

Closed by #777