tics: test should check tidb_enable_change_multi_schema before changing multi schema

[AilinKid]

Bug Report

Please answer these questions before submitting your issue. Thanks!

Since multi-schema-change is not a feature of release-5.0, we add a variable to prevent side effects from the stability of v5.0.
So tics test should be aware of that and make the tests be compatible with this variable.

1. Minimal reproduce step (Required)

https://internal.pingcap.net/idc-jenkins/blue/organizations/jenkins/tidb_ghpr_tics_test/detail/tidb_ghpr_tics_test/3240/pipeline

2. What did you expect to see? (Required)

Shouldn't be any literal character like Unsupported multi schema change

3. What did you see instead (Required)

Unsupported multi schema change

4. What is your TiDB version? (Required)

The variable is in tidb/pull/21533

[AilinKid]

PTAL @JaySon-Huang

[ti-srebot]

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[AilinKid]

duplicated with pingcap/tiflash#1271