util/testkit: make the msg of failed check more readable

[winoros]

What problem does this PR solve?

Currently the error msg when one explain checked failed is as following:

    res.c.Assert(got, check.Equals, need, res.comment)
... obtained string = "[[Projection_11 10.00 root 9_aux_0] [└─Apply_13 10.00 root left outer semi join, inner:StreamAgg_20, equal:[eq(test.t.c, count(*))]] [  ├─TableReader_15 10.00 root data:TableScan_14] [  │ └─TableScan_14 10.00 cop table:t, range:[-inf,+inf], keep order:false] [  └─StreamAgg_20 1.00 root funcs:count(1)] [    └─HashRightJoin_22 1.00 root inner join, inner:TableReader_25, equal:[eq(s.a, t1.a)]] [      ├─TableReader_25 1.00 root data:Selection_24] [      │ └─Selection_24 1.00 cop eq(s.a, test.t.a)] [      │   └─TableScan_23 10.00 cop table:s, range:[-inf,+inf], keep order:false] [      └─TableReader_27 10.00 root data:TableScan_26] [        └─TableScan_26 10.00 cop table:t1, range:[-inf,+inf], keep order:false]]"
... expected string = "[[Projection_11 10.00 root 9_aux_0] [└─Apply_12 10.00 root left outer semi join, inner:StreamAgg_20, equal:[eq(test.t.c, count(*))]] [  ├─TableReader_15 10.00 root data:TableScan_14] [  │ └─TableScan_14 10.00 cop table:t, range:[-inf,+inf], keep order:false] [  └─StreamAgg_20 1.00 root funcs:count(1)] [    └─HashRightJoin_22 1.00 root inner join, inner:TableReader_25, equal:[eq(s.a, t1.a)]] [      ├─TableReader_25 1.00 root data:Selection_24] [      │ └─Selection_24 1.00 cop eq(s.a, test.t.a)] [      │   └─TableScan_23 10.00 cop table:s, range:[-inf,+inf], keep order:false] [      └─TableReader_27 10.00 root data:TableScan_26] [        └─TableScan_26 10.00 cop table:t1, range:[-inf,+inf], keep order:false]]"
... sql:explain select t.c in (select count(*) from t s , t t1 where s.a = t.a and s.a = t1.a) from t;, args:[]

It's not easy to see where is wrong.

What is changed and how it works?

Make the error msg line by line.

    res.c.Assert(resBuff.String(), check.Equals, needBuff.String(), res.comment)
... obtained string = "" +
...     "[Projection_11 10.00 root 9_aux_0]\n" +
...     "[└─Apply_13 10.00 root left outer semi join, inner:StreamAgg_20, equal:[eq(test.t.c, count(*))]]\n" +
...     "[  ├─TableReader_15 10.00 root data:TableScan_14]\n" +
...     "[  │ └─TableScan_14 10.00 cop table:t, range:[-inf,+inf], keep order:false]\n" +
...     "[  └─StreamAgg_20 1.00 root funcs:count(1)]\n" +
...     "[    └─HashRightJoin_22 1.00 root inner join, inner:TableReader_25, equal:[eq(s.a, t1.a)]]\n" +
...     "[      ├─TableReader_25 1.00 root data:Selection_24]\n" +
...     "[      │ └─Selection_24 1.00 cop eq(s.a, test.t.a)]\n" +
...     "[      │   └─TableScan_23 10.00 cop table:s, range:[-inf,+inf], keep order:false]\n" +
...     "[      └─TableReader_27 10.00 root data:TableScan_26]\n" +
...     "[        └─TableScan_26 10.00 cop table:t1, range:[-inf,+inf], keep order:false]\n"
... expected string = "" +
...     "[Projection_11 10.00 root 9_aux_0]\n" +
...     "[└─Apply_12 10.00 root left outer semi join, inner:StreamAgg_20, equal:[eq(test.t.c, count(*))]]\n" +
...     "[  ├─TableReader_15 10.00 root data:TableScan_14]\n" +
...     "[  │ └─TableScan_14 10.00 cop table:t, range:[-inf,+inf], keep order:false]\n" +
...     "[  └─StreamAgg_20 1.00 root funcs:count(1)]\n" +
...     "[    └─HashRightJoin_22 1.00 root inner join, inner:TableReader_25, equal:[eq(s.a, t1.a)]]\n" +
...     "[      ├─TableReader_25 1.00 root data:Selection_24]\n" +
...     "[      │ └─Selection_24 1.00 cop eq(s.a, test.t.a)]\n" +
...     "[      │   └─TableScan_23 10.00 cop table:s, range:[-inf,+inf], keep order:false]\n" +
...     "[      └─TableReader_27 10.00 root data:TableScan_26]\n" +
...     "[        └─TableScan_26 10.00 cop table:t1, range:[-inf,+inf], keep order:false]\n"
... sql:explain select t.c in (select count(*) from t s , t t1 where s.a = t.a and s.a = t1.a) from t;, args:[]

Check List

Tests

• Existing tests.

Code changes

• Has exported function/method change

[alivxxx]

LGTM

[zz-jason]

LGTM

[zhexuany]

LGTM