Switch CreatePackage flag with new IsPackable flag

[lschoenepxc]

Summary

Delete old CreatePackage flag, because it was not working anymore
Add IsPackable false if necessary (IsPackable true is the default)
Added a Directory.Build.props to the src/Tests directory to disable packaging for all test-projects in the directory

Review

Typical tasks

• Merge-Request is well described
• Critical sections are documented in code
• Tests are extended
• Documentation is created / updated
• Running in test environment

Clean code

• All unused references are removed
• Clean code rules are respected with passion (naming, ...)
• Avoid copy and pasted code snippets