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GH-37294: [C++] Use std::string for HasSubstr matcher (#37314)



### Rationale for this change

Apparently not all versions of GTest/GMock handle `testing::HasSubstr` against a `std::string_view`.

### What changes are included in this PR?

Explicitly wrap in `std::string`.

### Are these changes tested?

They are the tests.

### Are there any user-facing changes?

No.
* Closes: #37294

Authored-by: David Li <li.davidm96@gmail.com>
Signed-off-by: Sutou Kouhei <kou@clear-code.com>