engine: Fix FileServletTest

- A user in engine-devel reported this unit test failing
  and we figured out it was because it was trying to read
  a file from the environment which was bigger than expected
  Fixed the test to read a file from the test resources
  instead.

Change-Id: I21794f1838e6161c422561f5023999d0e37f6a71
Signed-off-by: Alexander Wels <awels@redhat.com>