Mock storage space checks for tests.

Description

Mocks the call to storage information in order not to depend on the space available on the host's disk.

Changes proposed in this merge request:

• Mock calls to compute disk space and availability
• Add two tests when the configuration would fail

This merge request, when approved, will close issue/bug: #530 (closed) This is a new MR from !635 (closed) from the main BuildStream repo

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