Increase test coverage of FetchRequest.swift
Increase test coverage of FetchRequest.swift

Existing DatabaseValueConvertible tests were only testing the
`extension DatabaseValueConvertible where Self:
StatementColumnConvertible` cases.

Added tests to explicitly cover the cases where something conforms to
`DatabaseValueConvertible` but *not* `StatementColumnConvertible`, and
renamed the old tests.

groue/GRDB.swift,groue/GRDB.swift,zmeyc/GRDB.swift,zmeyc/GRDB.swift,groue/GRDB.swift,groue/GRDB.swift,zmeyc/GRDB.swift,zmeyc/GRDB.swift