adb: make test_device.py executable.

For symmetry with test_adb.py, since they have different interpreter
requirements now.

Test: ./test_device.py
Change-Id: I02fe659a7216a619383661c8019d356f9ccfb34d
diff --git a/adb/test_device.py b/adb/test_device.py
old mode 100644
new mode 100755