selftest: fib_tests: Always cleanup before exit

Usage of `set -e` before executing a command causes immediate exit
on failure, without cleanup up the resources allocated at setup.
This can affect the next tests that use the same resources,
leading to a chain of failures.

A simple fix is to always call cleanup function when the script exists.
This approach is already used by other existing tests.

Fixes: 1056691b

 ("selftests: fib_tests: Make test results more verbose")
Signed-off-by: Roxana Nicolescu <roxana.nicolescu@canonical.com>
Link: https://lore.kernel.org/r/20230220110400.26737-2-roxana.nicolescu@canonical.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>