Multi-byte SPI transfers - speeds up TFT support

[ukmaker]

Redrawing a TFT screen over SPI can be fairly slow. Things like flood-fills to clear an area of the screen use multiple calls totransfer()or transfer16(), which carry substantial overhead.

I have a couple of additional functions which allow for the same 8- or 16-bit values to be transferred multiple times with less overhead. This results in e.g. writing a block of colour using the Adafruit SPITFT library to be approx 3x faster. The practical result is that scrolling multiple lines of text now happens smoothly instead of blinking.

See PR: #1699

I'd like to run the CI suite against this but I'm not sure how. Are there any docs on how to do that?