[hardware][interfaces][vehicle] fix -Wdangling-gsl in test

Chaining the method calls in such a way otherwise returns a dangling
pointer to a temporary object.

Bug: 139945549
Test: mm
Change-Id: I0783fccbb6f11e7e37bd059445265227359649cf
Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>