[marathon] Use test builder.

This change introduces the builder pattern to the Marathon unit tests in
order to simplify and reduce the amount of testing boilerplate.

Additional changes:

- Add missing unit tests.
- Make all tests look consistent.
- Use dedicated type for task states for increased type safety.
- Remove obsoleted getApplication function.

testhelpers/helpers.go

@@ -12,6 +12,11 @@ func Intp(i int) *int {
 	return &i
 }
 
+// Stringp returns a pointer to the given string value.
+func Stringp(s string) *string {
+	return &s
+}
+
 // MustNewRequest creates a new http get request or panics if it can't
 func MustNewRequest(method, urlStr string, body io.Reader) *http.Request {
 	request, err := http.NewRequest(method, urlStr, body)