v0.20.0

Added

• Fixture existing_db_factory to directly use database from databases subdirectory.
  It's not intended for use in Firebird QA, but it's necessary for other plugin
  users.

Fixed

• Report test error also in cases when unexpected stderr is returned from tool execution
  while returncode is zero.
• Select test marked for current platform also when it's not marked for Firebird version.