d/tests/internaltest: use -e shell switch to detect unit tests errors

The libnftnl's tests-script.sh script returns the return code of the last
executed unit test.

We want to fail if any of the internal unit test result in errors.

Git-Dch: Ignore
Signed-off-by: Arturo Borrero Gonzalez <arturo.borrero.glez@gmail.com>