Improve and extend test_rewrite_older and test_recovery

[mulkieran]

Something like this would allow also to test behaviour on other differences besides time stamp:

• Set up 2 equal StaticHeaders
• Mutate them as appropriate, timestamp and other
• Write them out to a buffer
• Ensure StaticHeader::setup() does the correct thing.

proptest might be our friend here.

The difference between this test and bda_test_recovery is that that test randomly changes the buffers, not the headers. So, it tests only fixes at the level where one static header location is unreadable.

test_recovery tests the situation where both buffers are readable, but one or both are not correct StaticHeaders. It covers, at least in theory:

• Ok(Some(loc1)), Ok(Some(loc2)) where loc1 and loc2 are equal. _It omits the cases where these are unequal, which is what test_rewrite_older is supposed to cover._
• Ok(None), Ok(None) where both magics have been corrupted
• Ok(Some(loc1)), Ok(None) and the reverse where one magic is corrupted but the other is well-formed
• Ok(Some(loc1)), Err(...) and the reverse where one is well-formed and the other is ill-formed
• Ok(None), Err(...) and the reverse where one has magic corrupted and the other is ill-formed
• Err(...), Err(...) where both are ill-formed

Neither test covers an unreadable buffer.

[mulkieran]

It looks like the handy Rust crate partial-io might be our friend as well.

[mulkieran]

@GuillaumeGomez This one is very open-ended. I think it makes sense to try to explore the partial-io crate to see how if it can be used to help out w/ our testing problem - I only took a brief look at it, but it did seem a likely candidate.

[mulkieran]

@lleshchi You might want to think about this while you're working on https://github.com/stratis-storage/stratisd/issues/2133