jvpp: introducing callback api and future api tests for all plugins (VPP-591)

jvpp: introducing callback api and future api tests for all plugins (VPP-591)

test can be run with:
make test TEST=test_jvpp

memory_shared.c:
declaring and assigning variable in if statement
makes it usage outside statement impossible. Looks like memory
space assigned to variable declared in statement is freed when
statement ends
svm.c:
- fixed case when root path can have a "/" at beggining
- added option for test to operate over shared memory space
with /vpe-api name and not create new one with name consisting of
root path and region name which would require root permisions

Change-Id: Iff1170dc6a5c1be134c152f2757c7ab9b919a8ed
Signed-off-by: Matej Perina <mperina@cisco.com>