def-tool: Skip dex test if smali cannot be found

This commit skips dex test cases if the `smali` command cannot be found
from the path.  Before this commit, the test was not skipped because the
`subprocess.checked_call()` throws `OSError` (instead of `IOError`) in
Python 2.

Test: ./tests/run.sh
Change-Id: Ic7ec11ab9d9de2294132d88a2b084ef11dd01c87
1 file changed