server: add test for LOAD DATA

[imtbkcat]

What problem does this PR solve?

TiDB cannot support LOAD DATA sql which fields terminated by hex expression.

load data local infile '/home/pingcap/jeff-test/merchant.dmp' into table `merchant` fields terminated by X'01'
2018/06/29 13:29:22.709 conn.go:481: [warning] [8448] dispatch error:

parser pr: pingcap/parser#365

What is changed and how it works?

Add hexLit and binaryLit for FieldTerminator, and convert hex expression to string during parsing.

After:

mysql> load data local infile '~/1.csv' into table t_sp FIELDS TERMINATED BY X'2C2C2C2C' enclosed by '"';
Query OK, 3 rows affected (0.00 sec)
Records: 3  Deleted: 0  Skipped: 0  Warnings: 0

Check List

Tests

• Unit test

Code changes

• None

Side effects

• None

Related changes

• Need to cherry-pick to the release branch