Lightning: testcase contains incompatible go units

## Bug Report

Please answer these questions before submitting your issue. Thanks!

### 1. Minimal reproduce step (Required)

lightning_write_limit test

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

no TOML error

### 3. What did you see instead (Required)

CI error stack:
```
errorVerbose="invalid suffix: 'mi'
github.com/pingcap/errors.AddStack
/go/pkg/mod/github.com/pingcap/errors@v0.11.5-0.20240318064555-6bd07397691f/errors.go:178
github.com/pingcap/errors.Trace
/go/pkg/mod/github.com/pingcap/errors@v0.11.5-0.20240318064555-6bd07397691f/juju_adaptor.go:15
github.com/pingcap/tidb/br/pkg/lightning/config.(*Config).LoadFromTOML
/home/jenkins/agent/workspace/pingcap/tidb/release-7.5/pull_br_integration_test/tidb/br/pkg/lightning/config/config.go:1532
github.com/pingcap/tidb/br/pkg/lightning/config.(*Config).LoadFromGlobal
/home/jenkins/agent/workspace/pingcap/tidb/release-7.5/pull_br_integration_test/tidb/br/pkg/lightning/config/config.go:1494
github.com/pingcap/tidb/br/cmd/tidb-lightning.main.func2
/home/jenkins/agent/workspace/pingcap/tidb/release-7.5/pull_br_integration_test/tidb/br/cmd/tidb-lightning/main.go:91
github.com/pingcap/tidb/br/cmd/tidb-lightning.main
/home/jenkins/agent/workspace/pingcap/tidb/release-7.5/pull_br_integration_test/tidb/br/cmd/tidb-lightning/main.go:95
github.com/pingcap/tidb/br/cmd/tidb-lightning.TestRunMain.func2
/home/jenkins/agent/workspace/pingcap/tidb/release-7.5/pull_br_integration_test/tidb/br/cmd/tidb-lightning/main_test.go:45
runtime.goexit
/usr/local/go/src/runtime/asm_amd64.s:1650"
```

### 4. What is your TiDB version? (Required)

7.5.5