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GH-37601: [C++][Parquet] Add missing GoogleMock dependency (#37602)

### Rationale for this change

Parquet's tests uses GoogleMock. So we should specify GoogleMock as a dependency explicitly.

### What changes are included in this PR?

Add `${ARROW_GTEST_GMOCK}` to `PARQUET_MIN_TEST_LIBS`.

### Are these changes tested?

Yes.

### Are there any user-facing changes?

No.
* Closes: #37601

Authored-by: Sutou Kouhei <kou@clear-code.com>
Signed-off-by: Sutou Kouhei <kou@clear-code.com>