test behavior when there is no sandbox on Linux

Description

Ensure helpful messages are raised when there are no sandbox available on Linux

Removing some caching has some performance implication. I tried running 3 times every type (full cache removal, only platform, no cache removal) to compare.

With only platform removal, the penalty hit over the entire test suite is 2 to 5%. With the whole cache removal, the penalty hit is 8 to 10%.

I therefore think that only wiping the platform is a good first step, and only wiping the bwrap cache for tests that need it is simpler.

Changes proposed in this merge request:

• Erase platform between each test for better isolation
• Add tests about sandbox
• Erase bwrap cache only for these specific changes

This merge request, when approved, will close: #736 (closed) and #696 (closed)

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