Add INFIX_NOT_OPERATOR feature flag

Adds `!` as an alias of `NOT` with the precedence of a unary operator (i.e. higher). This matches MySQL's behavior.

Thanks to @nelgau for #110, for which this is the cleanup as discussed there.

Also add `.features` file support to NodeToStringTest by moving the SQLParserTest logic to the base. Allows more flexible unparser tests.