tests: work in VPATH setup again

tests: work in VPATH setup again

* defs: Drop overly paranoid sanity checks that was causing all the tests
to fail spuriously when run in a VPATH setup, with a message like:
"../t/nodef.sh: ./t/ax/test-init.sh: not found in current directory".
Those checks looked for invariants that, even if broken, would still
cause the test to fail very early and with a pretty clear error message
anyway.

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