Exercise stage-output extensions_to_txt in testing

This updates our stage-output test to exercise the extensions_to_txt
behavior. Do this as we've seen the performance of these tasks is quite
poor. Testing will help us improve this without breaking things.

Change-Id: I35844fa438773b4789e7f1e4d223bd59d5df7ba7

test-playbooks/stage-output.yaml

@@ -4,12 +4,28 @@
       copy:
         content: "Test file"
         dest: "/tmp/test-output.txt"
-    - name: Test stage-output with a file
+    - name: Write a second test file
+      copy:
+        content: "Second test file"
+        dest: "/tmp/test-output.log"
+    - name: Test stage-output with explicit files
       include_role:
         name: stage-output
       vars:
         zuul_copy_output:
           /tmp/test-output.txt: logs
+          /tmp/test-output.log: logs
+        extensions_to_txt:
+          log: true
+          txt: false
+    - name: Check the file was renamed
+      stat:
+        path: /home/zuul/logs/test-output_log.txt
+      register: renamed_file_stat
+    - name: Fail if file was not renamed
+      fail:
+        msg: "Renamed test-output log file does not exist."
+      when: not renamed_file_stat.stat.exists
     - name: Test stage-output with no input
       include_role:
         name: stage-output