Add unit tests

[EdgarScott]

Hi. I've analysed your codebase and noticed that it is not fully tested. I've written some tests for the methods in the following classes with the help of Diffblue Cover:

• qunar.tc.bistoury.clientside.common.meta.Numbers
• qunar.tc.decompiler.code.Instruction
• qunar.tc.decompiler.code.SwitchInstruction
• qunar.tc.decompiler.main.extern.IIdentifierRenamer_Type
• qunar.tc.decompiler.modules.renamer.ConverterHelper
• qunar.tc.decompiler.struct.attr.StructAnnDefaultAttribute
• qunar.tc.decompiler.struct.attr.StructAnnotationAttribute
• qunar.tc.decompiler.struct.attr.StructAnnotationParameterAttribute
• qunar.tc.decompiler.struct.attr.StructBootstrapMethodsAttribute
• qunar.tc.decompiler.struct.attr.StructConstantValueAttribute
• qunar.tc.decompiler.struct.attr.StructEnclosingMethodAttribute
• qunar.tc.decompiler.struct.attr.StructExceptionsAttribute
• qunar.tc.decompiler.struct.attr.StructGeneralAttribute
• qunar.tc.decompiler.struct.attr.StructGenericSignatureAttribute
• qunar.tc.decompiler.struct.attr.StructInnerClassesAttribute
• qunar.tc.decompiler.struct.attr.StructLineNumberTableAttribute
• qunar.tc.decompiler.struct.attr.StructMethodParametersAttribute
• qunar.tc.decompiler.struct.gen.VarType
• qunar.tc.decompiler.struct.gen.generics.GenericType
• qunar.tc.decompiler.util.InterpreterUtil
• qunar.tc.bistoury.instrument.client.spring.el.CodeFlow
• qunar.tc.bistoury.instrument.client.spring.el.CollectionUtils
• qunar.tc.bistoury.instrument.client.spring.el.NestedExceptionUtils
• qunar.tc.bistoury.instrument.client.spring.el.ObjectUtils
• qunar.tc.bistoury.instrument.client.spring.el.Operator_DescriptorComparison
• qunar.tc.bistoury.instrument.client.spring.el.StringUtils
• qunar.tc.bistoury.serverside.agile.Base64
• qunar.tc.bistoury.serverside.agile.Strings

Hopefully, these tests will help you detect any regressions caused by future code changes. If you would find it useful to have additional tests written for this repository, I would be more than happy to look at other classes that you consider important in a subsequent PR.