phablet-test-run doesn't check that image is writable

[Cris Dywan]

Using it to run test cases on a freshly flashed device won't work:

> phablet-test-run -p ubuntu-ui-tookit-autopilot ubuntuuitoolkit
adbd is already running as root
W: Not using locking for read only lock file /var/lib/dpkg/lock
E: Unable to write to /var/cache/apt/
E: The package lists or status file could not be parsed or opened.
bash: cd: /home/phablet/autopilot: No such file or directory
Traceback (most recent call last):
File "/usr/bin/autopilot", line 352, in <module>
main()
File "/usr/bin/autopilot", line 343, in main
run_tests(args)
File "/usr/bin/autopilot", line 86, in run_tests
test_suite = load_test_suite_from_name(args.suite)
File "/usr/bin/autopilot", line 229, in load_test_suite_from_name
package = __import__(top_level_pkg)
ImportError: No module named ubuntuuitoolkit
/usr/bin/phablet-test-run: 107: return: Illegal number: 1

It would help if phablet-test-run could handle making the image writable because currently a lot of time is spent on typos or forgetting to touch the magic file etc. Ideally one command would be enough to run the tests on a new image.