[Tests] : Write more tests on AVR8GCC toolchains and related components

[bebenlebricolo]

Many components are not properly tested at the moment. Add more tests in order to validate that the data model provided by CMake core is correctly converted into a valid representation for Atmel Studio 7.0.

This includes :

• Small unit tests
• Intermediate tests levels where we check the produced model matches the requirements
• More global workflow with real Xml generation for manual QC and/or parsing to validate the constructed model
• Complete workflows using the CMakeAS7-TestsProjects repository.

Concerned files and components :

• [AS7DeviceResolver.h]
• [AS7ToolchainTranslator.h]
• [AvrGCC8Toolchain.h]
• [AS7DeviceResolver.h]
• [cmAvrGccCompiler.h]
• [cmAvrGccCompilerOption.h]
• [cmAvrGccDebugOption.h]
• [cmAvrGccCompilerOption.h]
• [cmAvrGccDefinitionOption.h]
• [cmAvrGccLanguageStandardOption.h]
• [cmAvrGccLinkerOption.h]
• [cmAvrGccMachineOption.h]
• [cmAvrGccOptimizationOption.h]
• [cmAvrGccWarningOption.h]

AtmelStudio7 generators would be nice as well, but they will be much harder to test because of CMake's inherited dependency.