unit: Add SPDX License Identifier

unit: Add SPDX License Identifier

This patch adds SPDX License Identifier and removes the license text.

-------------------------------------
       License            COUNT
-------------------------------------
 GPL-2.0-or-later     :     25
 LGPL-2.1-or-later    :      1
 GPL-2.0-only         :      2
 BSD-2-Clause         :      1

License: GPL-2.0-or-later
   unit/test-hog.c
   unit/test-eir.c
   unit/test-gobex-header.c
   unit/test-crc.c
   unit/test-avrcp.c
   unit/test-ringbuf.c
   unit/test-gobex-apparam.c
   unit/test-gobex.c
   unit/test-hfp.c
   unit/test-textfile.c
   unit/test-avdtp.c
   unit/test-sdp.c
   unit/test-gatt.c
   unit/test-avctp.c
   unit/test-gattrib.c
   unit/test-queue.c
   unit/test-gobex-packet.c
   unit/test-uuid.c
   unit/test-crypto.c
   unit/test-uhid.c
   unit/test-gdbus-client.c
   unit/test-midi.c
   unit/test-lib.c
   unit/test-mgmt.c
   unit/test-gobex-transfer.c

License: LGPL-2.1-or-later
   unit/test-mesh-crypto.c

License: GPL-2.0-only
   unit/util.h
   unit/util.c

License: BSD-2-Clause
   unit/test-ecc.c

Signed-off-by: Anuj Jain <anuj01.jain@samsung.com>
Signed-off-by: Ayush Garg <ayush.garg@samsung.com>