The `test` of `ConditionalExpression` can not be an unparenthesized `ArrowExpression`

[JLHwung]

() => {} ? 1 : 0

Expected: Acorn throws syntax error. This is invalid per https://tc39.es/ecma262/#prod-AssignmentExpression, the test of a ConditionalExpression is a ShortCircuitExpression, which does not cover ArrowFunctionExpression.

Acutal: It parses successfully.

[marijnh]

Attached patch adds a check for this.