test/{ipc,uipc}: Fix the path to the JSON schema validator

test/{ipc,uipc}: Fix the path to the JSON schema validator

The test specs for the ipc and uipc modules assume to be executed
in the test directory. Thus, they call the JSON schema validator
using a path that is relative to the test directory. However, the
test specs should assume to be executed in the root directory of
the source tree.

This commit changes the paths to the JSON schema validator in the
test specs for the ipc and uipc modules so they are relative to
the root of the source tree.