Create test descriptions using a helper

Instead of creating test descriptions via SystemDescription#from_json,
let's create them using a helper. This creates a layer between the tests
and SystemDescription#from_json, making things like changing method
parametrs and filling-in uninteresting parameters easier. For example,
we'll be able to omit irrelevant description names and make #from_json's
storage parameter required easily.

Note the change is 100% mechanical -- created descriptions are exactly
as they were before.

Also note I didn't convert #from_json calls where the method itself, or
some code called from it, was the subject of testing.
Don't name test description whose names aren't relevant

The existence of the create_test_description helper allows us not to
explicitly name tests descriptions whose names aren't relevant. This
reduces clutter a bit.
Add file store method to rename
Use tmp directory for extracting unmanaged files

During inspection unmanaged files are extracted to the file store
"unmanaged_files.tmp".
The directory gets renamed as soon as the inspection is finished.

This allows debugging after canceling the inspection and also prevents
overwriting of working data.

If unmanaged_files were extracted before the file store is kept until
the new inspection is finished to keep the system description state
consistent.

Fixes #160
Mention tmp directory during unmanaged files inspection cancelation

Unmanaged files are now extracted to a temporary directory. If the user
cancels the process he gets a message where these partly extracted files
are available.
Merge pull request #204 from SUSE/system-description-always-have-store

[Review] Request from 'dmajda' @ 'SUSE/machinery/system-description-always-have-store'
Merge pull request #207 from SUSE/extract_unmanaged_files_to_tmp

Extract unmanaged files to tmp
Add create_test_description to integration tests

The helper method create_test_description was only available for the unit
tests, not the integration tests.
Move create_test_description to its own file

Created spec/helper/shared_test_methods.rb a place for methods which are
used for unit and integration tests at the same time.

That prevents us from duplicating the code for create_test_description.