`cast_string_string` in TiFlash standalone test has incorrect behaviour.

note that this issue won't reproduce with tidb, it only occurs in tiflash standalone.

reproduce step

1. start a standalone tiflash:

./tiflash server --config=<tics>/tests/docker/config/tics_dt.toml

2. create a test file, named cast_string_string_debug.test

# Preparation.
=> DBGInvoke __enable_schema_sync_service('true')

=> DBGInvoke __drop_tidb_table(default, test)
=> drop table if exists default.test

=> DBGInvoke __set_flush_threshold(1000000, 1000000)

# Data.
=> DBGInvoke __mock_tidb_table(default, test, 'a char')
=> DBGInvoke __refresh_schemas()
=> DBGInvoke __put_region(4, 0, 100, default, test)
=> DBGInvoke __raft_insert_row(default, test, 4, 50, '123d')

=> DBGInvoke dag('select count(1) from default.test group by a, cast_string_string(a)', 4, 'encode_type:chunk')
┌─count(1)─┬─a────┬─cast_string_string(a)─┐
│        1 │ 123d │                  123d │
└──────────┴──────┴───────────────────────┘

# Clean up.
=> DBGInvoke __drop_tidb_table(default, test)
=> drop table if exists default.test

3. run this test:

<tics>/tests/run-test.sh cast_string_string_debug.test

File: delta-merge-test/query/expr/cast_string_string_debug.test
Error line: 15
Error: DBGInvoke dag('select count(1) from default.test group by a, cast_string_string(a)', 4, 'encode_type:chunk')
Result:
  ┌─count(1)─┬─a────┬─cast_string_string(a)─┐
  │        1 │ 123d │                   123 │
  └──────────┴──────┴───────────────────────┘
Expected:
  ┌─count(1)─┬─a────┬─cast_string_string(a)─┐
  │        1 │ 123d │                  123d │
  └──────────┴──────┴───────────────────────┘

4. tiflash version: master

/Users/lf/flash/tiflash/storage/ch/cmake-build-qz/dbms/src/Server/tiflash version
TiFlash
Release Version: v5.0.0-nightly-52-g2be501e2b-dirty
Edition:         Community
Git Commit Hash: 2be501e2b7ce1529b654e36f22aa3708a16e9c0b
Git Branch:      master
UTC Build Time:  2021-04-15 05:19:55
Profile:         DEBUG

Raft Proxy
Git Commit Hash:   5a8ab3893eebf88c0cf0727a958a4c63f0f29df8
Git Commit Branch: HEAD
UTC Build Time:    2021-04-12 09:53:32
Rust Version:      rustc 1.51.0-nightly (1d0d76f8d 2021-01-24)
Storage Engine:    tiflash
Prometheus Prefix: tiflash_proxy_
Profile:           debug

Start by running tests/run-test.sh with the provided cast_string_string_debug.test scenario in delta-merge-test/query/expr. Trace the cast_string_string entry point and compare the standalone TiFlash result with the expected output; done means the test preserves the input value 123d rather than producing 123.