tests/lint: rework ATF test driver

To skip tests that work only on particular platforms, a simple 'eval'
from the shell does not work since it does not exit on failure.  Fix
this by storing the commands in a local variable first.

Remove configuration knobs 'lint1-only-if-arch' and 'lint1-skip-if-arch'
since they are unused.

When skipping a test, actually mark it as skipped in the ATF statistics.

(rillig)