tests/checkasm: switch to external checkasm

There are a number of benefits tied to the upstream / third-party checkasm
version, including:

- Improved long-term maintainability, code reuse with other projects, etc.

- Vastly improved overall performance / runtime for benchmarking, due
  primarily to the ability to scale the runtime of each test to that test's
  complexity.

- Much more robust statistical analysis of benchmarking results; including
  robust outlier rejection, an estimation of the histogram, and the ability
  to report the variance / stddev in addition to the (trimmed) mean.

- Interactive HTML and JSON output formats in addition to CSV/TSV.

- More readable and user-friendly output across the board, especially for
  failures and data dumps (e.g. also showing errors inside padding bytes).

- Better cross-platform support, including dynamic fallback of timer
  implementations on ARM platforms, a better RISC-V harness, and more.

There are multiple approaches to how we can solve the problem o...