Add TestBase#sqlAndExpected(File dir, String extraExt) for flexibility.

Move parseFeatures to TestBase.

Use both of these from SQLParserTest and NodeToStringTest.
Add INFIX_NOT_OPERATOR feature flag.

Supports ! as a unary expression for NOT. Has a higher precedence
but is otherwise equivalent. This matches the MySQL ! behavior.

A slight tweak of a proposal from @nelgau to be slightly more
consistent with other feature flag behavior.

Fixes #105
merge nathanlws: Add INFIX_NOT_OPERATOR feature flag

Merge pull request #112 from nathanlws/sql-parser/infix-not-feature-flag
https://github.com/FoundationDB/sql-parser/pull/112