fix issue where otest-query will accidentally start running tests.

See comment inside for more info.  The gist of it is... a lingering
value stored in the 'SenTest' or 'XCTest' preference can make
otest-query accidentally _run_ tests when all it wanted to do was
_query_ the list of tests.
Merge pull request #298 from facebook/fpotter/otest-query-fix

fix issue where otest-query will accidentally start running tests.