unit: Add test for gatt_exchange_mtu()

The new unit/test-gatt.c file will contain tests for GATT API
implemented in attrib/gatt.{c,h}.
unit: Add test for gatt_discover_primary()
unit: Add test for gatt_discover_char_desc()
unit: Add test for gatt_write_cmd()
unit: Add test for gatt_write_char()
unit: Add test for gatt_read_char_by_uuid()
unit: Add test for gatt_read_char()
unit: Add test for long read at gatt_read_char()
unit: Add test for gatt_discover_primary() (by UUID)
unit: Add test for long write using gatt_write_char()
unit: Add test for gatt_discover_char()
unit: Add test for gatt_discover_char() (by UUID)
unit: Add test for gatt_find_included()
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