test: add embedded-test integration (WIP)

[kaspar030]

Description

This PR intents to provide a seamless integration of embedded-test.

This is very WIP, basically I'm in the experimentation phase.
This is taking shape. Still waiting on a probe-rs release, though.

This can be tested with probe-rs-tools from git (not 0.24.0) as follows:

#  laze -Cexamples/testing b -v -b nrf52840dk -s executor-thread cargo-test

Issues/PRs references

Fixes #331.
Depends on #506, #509.

Open Questions

• needs probe-rs from git due to Fix semihosting command caching for arm probe-rs/probe-rs#2604
• embedded-test allows both async and sync tests. this integration starts a thread, and inside the thread, a thread executor. that means the test cannot use non-Send stuff from the system executor (e.g., network stack).
  so, we need a way to re-use the system executor.
• integration is hacked into embedded-test fork, needs cleanup (& upstream PR)
• document ...

Change checklist

• I have cleaned up my commit history and squashed fixup commits.
• I have followed the Coding Conventions.
• I have performed a self-review of my own code.
• I have made corresponding changes to the documentation.