arrow

GH-45423: [C++] Don't require Boost library with ARROW_TESTING=ON/ARROW_BUILD_SHARED=OFF (#45424)

### Rationale for this change

We can use `libarrow_testing.a` without `boost::filesystem` when `arrow::util::Process` isn't used.

### What changes are included in this PR?

`ARROW_TESTING=ON` requires Boost. If `ARROW_BUILD_SHARED=ON` is also used, Boost libraries are also required.

### Are these changes tested?

Yes.

### Are there any user-facing changes?

No.
* GitHub Issue: #45423

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