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GH-44911: [C#] Choose port numbers dynamically for ArrowStreamWriterTests (#44912)

### What changes are included in this PR?

`ArrowStreamWriterTests.CanWriteToNetworkStream` and `ArrowStreamWriterTests.CanWriteToNetworkStreamAsync` pick ports dynamically instead of hardcoding a static port.

### Are these changes tested?

Yes.

### Are there any user-facing changes?

No.

Resolves #44911.
* GitHub Issue: #44911

Authored-by: Curt Hagenlocher <curt@hagenlocher.org>
Signed-off-by: Curt Hagenlocher <curt@hagenlocher.org>