Document test infrastructure somewhere on web site

[stevegt]

This description of the test infrastructure from @dkegel-fastly is worth gold -- is it already documented somewhere on the web site, and if not, where should it go?

I first looked for it in the "testing" section of the Contributing page and would expect to see it near there -- right below that would be my vote.

Build-only tests for all small targets:
The "make smoketest" at the end of .github/workflows/linux.yml make sure that at least one example compiles for each target. 

Emulated tests for small targets:
The "make tinygo-baremetal" at the end of .github/workflows/linux.yml makes sure that very small set of tests (currently just encoding/hex) passes on a very small set of emulated targets (currently cortex-m-qemu). 

Real hardware tests for small targets:
Look at the test results for the tip of dev and release.  
The "TinyHCI" test results are from https://github.com/tinygo-org/tinyhci, which runs on real hardware.
I haven't looked at that at all, but I think it only tests hardware functions.

All of the above could probably stand some expansion.  In particular, tinygo-baremetal could make sure a simple smoke test of fmt passes.  ( Once reflect improves, perhaps after https://github.com/tinygo-org/tinygo/pull/2640 lands, we could even make sure "tinygo test fmt" passes.)

Originally posted by @dkegel-fastly in #254 (comment)