MINOR: [C++] Bump arrow-testing to include repro file for apache#41044 (apache#41139)

### What changes are included in this PR?

Bump `arrow-testing` to include the latest commit.

Authored-by: Felipe Oliveira Carvalho <[email protected]>
Signed-off-by: Antoine Pitrou <[email protected]>

Author: felipecrv

.github/workflows/cpp.yml

@@ -31,6 +31,7 @@ on:
       - 'cpp/**'
       - 'docker-compose.yml'
       - 'format/Flight.proto'
+      - 'testing'
   pull_request:
     paths:
       - '.github/workflows/cpp.yml'
@@ -44,6 +45,7 @@ on:
       - 'cpp/**'
       - 'docker-compose.yml'
       - 'format/Flight.proto'
+      - 'testing'
 
 concurrency:
   group: ${{ github.repository }}-${{ github.head_ref || github.sha }}-${{ github.workflow }}