Add coverage support for txtar

[moul]

We can compute coverage for binaries, checkout https://go.dev/testing/coverage/.

To post the coverage results on the CI, you can follow these steps:

1. Build and run binaries in txtar with the appropriate flags.
2. Aggregate the coverage files from different runtimes.
3. Conditionally perform this process only for the latest version of Go.
4. For the previous version (latest-1), perform traditional testing without coverage.

Related with #1215