When VDBE_PROFILE is enabled, ignore negative opcode times, which sometimes
occur, perhaps due to context swaps.
Make sure ORDER BY clauses on joins that involve virtual tables and that
have a DISTINCT clause work correctly.  This is a candidate fix for
ticket [388d01d4bb8f9].  Test cases for that ticket will be checked in
separately.
Add test cases to ensure correct operation of joins with a virtual table
that include DISTINCT and ORDER BY clauses.  Verification for ticket
[388d01d4bb8f9].