Add tests that "convert" SIARD to itself

[chalkos]

This ensures that the SIARD (1 and 2) modules do not discard any data.

Tests required (checking equality between first and last state):

• [ ] Convert SIARD1 (in-memory) -> SIARD1 (in-memory)
• [ ] Convert SIARD1 (in-memory) -> SIARD1 (in-memory) -> SIARD1 (in-memory)
• [ ] Convert SIARD2 (in-memory) -> SIARD2 (in-memory)
• [ ] Convert SIARD2 (in-memory) -> SIARD2 (in-memory) -> SIARD2 (in-memory)
• [ ] Convert SIARD2 (in-memory) -> SIARD2 (in-memory) -> SIARD2 (in-memory)
• [ ] Convert SIARD1 (in-memory) -> SIARD1 (file) -> SIARD1 (file) -> SIARD1 (file) -> SIARD1 (in-memory)
• [ ] Convert SIARD2 (in-memory) -> SIARD2 (file) -> SIARD2 (file) -> SIARD2 (file) -> SIARD2 (in-memory)
• [ ] Convert SIARD1 (in-memory) -> SIARD1 (file) -> SIARD2 (file) -> SIARD1 (file) -> SIARD1 (in-memory)
• [ ] Convert SIARD2 (in-memory) -> SIARD2 (file) -> SIARD1 (file) -> SIARD2 (file) -> SIARD2 (in-memory)