Fix bug when deleting from flash and add unit tests

[ali1234]

The terminating null should be sent after the offset. Sending it before causes the offset to be 256x too big, because it is little endian.

Also adds a log message, not shown by default.

[ali1234]

I think this is right. The firmware CDC parser code is not very clear about where null terminators are supposed to be, but other commands that have parameters don't have a null between the 32BLxxxx and the first parameter, and they do have one at the end.

[Daft-Freak]

Not sure the null is even needed for the commands that don't send strings, but all of them are doing it 🤔

Also, looks like this has always been wrong, no idea how I didn't notice that.

[ali1234]

I thought I had used this successfully in the past, but it does look like that extra null was always sent. Maybe something changed on firmware side.

[Gadgetoid]

Need to fix py.test (who even came up with that absurd namespacing horror) to pytest in the CI. I'd better sort that.

[ali1234]

I already sent a PR for that too.

[ali1234]

Wrote some unit tests for this, and found a bug in the process.

[ali1234]

Rebased so the tests can run.

[ali1234]

Hmm okay well that clearly needs some work.

[coveralls]

Pull Request Test Coverage Report for Build 4365346862

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage increased (+5.7%) to 86.102%

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Files with Coverage Reduction	New Missed Lines	%
ttblit/core/blitserial.py	1	92.37%

Totals
Change from base Build 4363815723:	5.7%
Covered Lines:	1270
Relevant Lines:	1475

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💛 - Coveralls

[Gadgetoid]

Hoorah! Tests! Thank you.