tests: fix automatic re-execution of tests with Zsh

tests: fix automatic re-execution of tests with Zsh

* defs: Use '$argv0' instead of '$0'.  With Zsh not started right
away in Bourne-compatibility mode, the latter will be the path not
of the test script itself, but of the file it's currently sourcing
-- i.e., in our case, './defs'.  This would cause the automatic
re-execution code to execute './defs' (basically a no-op) rather
than re-run the test correctly.

Signed-off-by: Stefano Lattarini <stefano.lattarini@gmail.com>