test: kdbus: Replace the code that accesses the attr with the cynara API

test: kdbus: Replace the code that accesses the attr with the cynara API

Replace the code that directly accesses /proc/self/attr/current
with a cynara API called cynara_creds_self_get_client.
This is because, direct accessing to /proc/self/attr/current is invalid when SMACK is disabled.

Change-Id: I844b7faf5488a925eea7b2e7129af8d71927f262
Signed-off-by: Unsung Lee <unsung.lee@samsung.com>