executor,sessionctx: refine output format for tidb-specific variables of boolean type (#11239)

[sre-bot]

cherry-pick #11239 to release-2.1

---

What problem does this PR solve?

Currently, the result of selecting tidb-specific variables of boolean type is not well formatted. I think we'd better make it same style with MySQL. For example:

tidb> set @@tidb_batch_delete='On';
Query OK, 0 rows affected (0.00 sec)

tidb> select @@tidb_batch_delete;
+---------------------+
| @@tidb_batch_delete |
+---------------------+
| On                  |
+---------------------+
1 row in set (0.00 sec)

tidb> set @@tidb_enable_streaming='OfF'
    -> ;
Query OK, 0 rows affected (0.00 sec)

tidb> select @@tidb_enable_streaming;
+-------------------------+
| @@tidb_enable_streaming |
+-------------------------+
| OfF                     |
+-------------------------+
1 row in set (0.00 sec)

tidb> set @@tidb_disable_txn_auto_retry=on;
Query OK, 0 rows affected (0.00 sec)

tidb> select @@tidb_disable_txn_auto_retry;
+-------------------------------+
| @@tidb_disable_txn_auto_retry |
+-------------------------------+
| ON                            |
+-------------------------------+
1 row in set (0.00 sec)

tidb> set @@tidb_batch_insert=0;
Query OK, 0 rows affected (0.00 sec)

tidb> select @@tidb_batch_insert;
+---------------------+
| @@tidb_batch_insert |
+---------------------+
| 0                   |
+---------------------+
1 row in set (0.00 sec)

What is changed and how it works?

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)

Code changes

N / A

Side effects

N / A

Related changes

N / A

[sre-bot]

/run-all-tests

[XuHuaiyu]

@wshwsh12 Please resolve the conlicts

[zz-jason]

LGTM

[XuHuaiyu]

LGTM

[sre-bot]

Your auto merge job has been accepted, waiting for 16307

[sre-bot]

/run-all-tests