tests: fix fileiotest
tests: fix fileiotest

engine() cannot be called before the local_engine was constructed because it
dereferences the pointer local_engine to create a reference.
Consequently, the following error can be seen while running fileiotest:
./core/reactor.hh:854:13: runtime error: reference binding to null pointer of
type 'struct reactor' ASAN:SIGSEGV

Let's switch the test to use app_template.

Signed-off-by: Raphael S. Carvalho <1e2942cea40e3b4d2162dfcd9e53e41c05a84d22@cloudius-systems.com>

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