Support for don't care writes in I2C

[simonsso]

Motivation

I have a test case where there are a ton of I²C writes which I don't care about in this specific test. I still want to mock execute them but the data written is not relevant.

Implementation

I have some ideas how this could be implemented.

1. The impl Transaction block 1 could be extended with a function write_ignore then either implemented by extending the enum Mode2 with WriteIgnore or a new field in the Transaction struct3

2. Another way would be to add an option to write()4 but that would break all tests for every user of the I²C whether they want this feature or not, so I don't think that is the path forward.

3. The same builder style interface as with_error5 could be used to modify Transaction

• Are there any negative implications I did not consider?
• Any preferred implementation style?

[2bndy5]

I wondered this as well when testing a library that does a lot of init work over SPI. I ended up just parametrizing a reusable test function for repeated init work.

[simonsso]

I made two implementations of this, I liked they way number 3 above turned out so I posted it in PR #136