tests: Coverage and cleanup/speedup

- Improved test coverage to >80% of lines, functions
  - When doctests are included.
  - Wrote new unit tests:
    - Explicit tests for invalid instructions in the
      ranges {`5xyn`, `8xyn`, `9xyn`, `Fxbb`}
    - `rand` Tests for 1052671 cycles, to ensure
      randomly generated number is < ANDed byte
    - `Ex9E` (sek), `ExA1`(snek) will press only the expected key,
      then every key except the expected key, for every address
    - `Fx0A` (waitk) asserts based on the waveform of a keypress.
      After all, an A press is an A press.
- Improved test performance by printing slightly less
- Removed nightly requirement
  - (now optional, with feature = "unstable")
- Amended justfile to test with `cargo nextest` (nice)
- Changed release builds to optlevel 3

src/cpu/tests/roms/jumptest.ch8

@@ -0,0 +1,2 @@
+
+ "$&(*,.02468:<>@BDFHJLNPRTVXZ\^`