tests.cmdlib: Test index operations

Test if the GetIndexFromIdentifier and InsertItemToIndex functions work
properly.
tests.opcodes/client: Add test for constants

Also, test if the ConvertNicDiskModifications function operates as
expected.
tests.config: Test attach/detach wrappers

Test if the AttachInstanceDisk and DetachInstanceDisk wrappers update
the configuration as expected.
tests.cmdlib: Check if attach/detach checks work

Also, update the mocked config to create orphan disks for attach
purposes.
tests.cmdlib: Extend tests for _ApplyContainerMods

Test if _ApplyContainerMods works properly in attach/detach scenarios.