Fix errors reported by rider

[stefannikolei]

• Used collection expressions were possible
• Added the used type were needed for clarification
• Disabled an Resharper rule With ImageSharp.sln.DotSettings --> It said that the Attribute ConstantExpected is not valid on floats
• Removed unused TestHelper
• Rewrote some tests to use utf8 string instead of raw byte array to make the test clearer