Fix a false negative in lint for "xsprintf()"-family functions

Summary:
Ref T13577. This lint rule correctly detects the error in pht('x %s y') but the narrow test for n_STRING_SCALAR prevents it from detecting the error in pht('x %s y'.'z').

Make the test broader.

Test Plan:

• Ran arc lint on HTTPSFuture.php, got a detection of the issue in T13577.
• Added a failing test and made it pass.

Maniphest Tasks: T13577

Differential Revision: https://secure.phabricator.com/D21453