test/iscsi: move g_spdk_iscsi definition to individual tests
Defining g_spdk_iscsi in test/lib/iscsi/common.c leads to duplicate
definitions in the pdu unit test.
Change-Id: I280d11a64a923fea2f2c0cb1dd391449b9a471fb
Signed-off-by: Daniel Verkamp <daniel.verkamp@intel.com>