TestMPPHintsScope is too slow

[hawkingrei]

Bug Report

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1. Minimal reproduce step (Required)

The unit tests for TiDB actually can't utilize TiFlash, but this query has been supported in a hackish way, and the result is that it's very slow.

--- PASS: TestMPPHintsScope (108.88s)
    --- PASS: TestMPPHintsScope/off (55.10s)
    --- PASS: TestMPPHintsScope/on (53.77s)

[2025/12/30 19:48:14.278 +08:00] [WARN] [tables.go:2806] ["rpc error: code = Unavailable desc = connection error: desc = \"error reading server preface: EOF\""]
[2025/12/30 19:48:14.278 +08:00] [WARN] [tables.go:2806] ["rpc error: code = Unavailable desc = connection error: desc = \"error reading server preface: EOF\""]

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

3. What did you see instead (Required)

4. What is your TiDB version? (Required)