shelltestrunner was already installed for the coverage target, but not
for main test ones (for the same reason that release 1.5 was broken,
in the sense that it was not needed…).
Switch to more recent versions of the checkout and codecov action, the
latter also removing saving the data locally, so remove the archival
of that data.
The embedding of tests in the Makefile works only for that is there
today - a few `--version` and `stinit -p` calls. For expanding the
test coverage, as much as it can be done for `mt`, let's use a proper
shell test runner. `shelltestrunner` is available in Debian/Ubuntu, so
it's likely to be available in other distributions too, and as such
should not be a big barrier to contribution.
