Libraries vary on whether they accept a no-argument println(),
but they all accept a single string value.

There aren't any standard mocks for these and our code
invokes the adafruit LittleFS library and RTC library
calls which assume they exist.

Doesn't really add any value over `pio test -e native` but stays
consistent with the current build procedure and any -D defines that
may become mandatory later.

Uses the native platform, which compiles with the host system's
compilers.  This was tested on MacOS' GNU c++.

Use the skaygin/ArduinoNative libdep for the Arduino platform
library dependencies; it's the most complete native mock
currently available in PlatformIO and covers almost everything
we use.

Reuses the preexisting stm32 clone of Adafruit's LittleFS for our
filesystem dependency, which seemed to be the easiest path
(though it's also the most demanding of the SPI interface.) Uses
private mocks of SPI and Wire.

The initial two sets of unit tests are fairly trivial, covering
our SHA256 library and mesh::Utils::toHex(), just to show that
everything's working.

Found the first bug, in fromHex() :)

- printHex (demonstrates a mock/spy stream)
- 2-part sha256

It's fast enough (and the current set of tests is small enough) to turn
on for the main native env, but not all local compilers will support it
so I made it a separate target.

Usage: pio test -e native-asan

Current tests all pass asan checks.

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