Add zuul_work_dir to run-test-command

To make it so that a repo can host a simple command which is an
integration test of multiple projects, add the semi-standard
zuul_work_dir variable to the job.

Change-Id: I265469ac9c12c3a38a11b243f3ff747d27141f21

playbooks/run-test-command/run.yaml

@@ -14,5 +14,5 @@
     - name: Run test_command
       command: '{{ item }}'
       args:
-        chdir: '{{ zuul.project.src_dir }}'
+        chdir: '{{ zuul_work_dir | default(zuul.project.src_dir) }}'
       with_items: '{{ test_commands }}'

zuul.d/general-jobs.yaml

@@ -36,6 +36,11 @@
       To use this, set the ``test_command`` variable to a single command or
       a list of commands. For complex list of commands, it is recommended to
       use a single command that runs a proper script.
+
+      .. zuul:jobvar:: zuul_work_dir
+         :default: {{ zuul.project.src_dir }}
+
+         Path to operate in.
     run: playbooks/run-test-command/run.yaml
     vars:
       test_command: "exit 1"