GH-49834: [C++] Avoid building re2 unit tests (#50152)

### Rationale for this change

We don't need to build unit tests for the bundled RE2 library, and it avoids `ctest` picking those tests when run from the build directory.

### Are these changes tested?

Manually.

### Are there any user-facing changes?

No.

* GitHub Issue: #49834

Authored-by: Antoine Pitrou <antoine@python.org>
Signed-off-by: Sutou Kouhei <kou@clear-code.com>

Author: pitrou

cpp/cmake_modules/ThirdpartyToolchain.cmake

@@ -3064,6 +3064,7 @@ function(build_re2)
 
   # Unity build causes some build errors
   set(CMAKE_UNITY_BUILD OFF)
+  set(RE2_BUILD_TESTING OFF)
 
   fetchcontent_makeavailable(re2)