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GH-35932: [Java] Make JDBC test less brittle (#35940)

### Rationale for this change

A JDBC test is brittle because it compares string representations.

### What changes are included in this PR?

Compare values directly.

### Are these changes tested?

N/A

### Are there any user-facing changes?

No.
* Closes: #35932

Authored-by: David Li <li.davidm96@gmail.com>
Signed-off-by: Raúl Cumplido <raulcumplido@gmail.com>