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GH-36090: [C++] Add testing libraries for Acero & Datasets (#36206)

### Rationale for this change

The acero & datasets tests rely on some common sources.  The current build includes those in every test executable as sources.  This was a bit of a change introduced as we extracted acero into its own library.  This causes those files to be built again and again and this caused an increase in build times.

This change creates test libraries for each of those modules.  These test libraries include the common sources and are built once.  These libraries can then be included in the tests.  This avoids the duplicate builds.

### What changes are included in this PR?

Changes to how common test sources are built and linked.

### Are these changes tested?

Yes.  These tests still run as part of the unit tests.

I'm not entirely sure if the cmake.in and pc.in files are tested or not as I don't know entirely how they factor into things.  However, I believe these are only needed if users want to rely on these test libraries and I'm not sure how critical that is.

### Are there any user-facing changes?

No.

* Closes GH-36090
* Closes: #36090

Authored-by: Weston Pace <weston.pace@gmail.com>
Signed-off-by: Sutou Kouhei <kou@clear-code.com>