testsuite: Skip gtkd test on non-Linux platform ...

This currently fails on Mac because the '.so' is hardcoded.
It should eventually be fixed, but for the time being,
my objective is to get things to pass on my machine.

replace all usages of `exit` ...

- those skipped the ERR trap
- add die function to exit with line number and message

less verbose issue782 test

fix failures (due to bash error flags)

unify flags and error handling ...

- also add trap ERR handler to report exact error location

Add test case for issue #782