explicit error when test_command is undef

Don't use a shell builtin: shell builtins aren't available since
'command' ansible module is used. Instead, fail with an explicit
error when the required test_command variable is not set.

Fix this error:

    Ansible output: b'failed: [host] (item=exit 1) => {
        "ansible_loop_var": "item",
        "changed": false,
        "cmd": "exit 1",
        "item": "exit 1",
        "msg": "[Errno 2] No such file or directory: 'exit': 'exit'",
        "rc": 2
    }

Change-Id: I88303f7302d7354ffc8b18e607b28349a9860a57

playbooks/run-test-command/run.yaml

@@ -1,6 +1,11 @@
 - hosts: all
 
   tasks:
+    - name: Abort when test_command variable is undefined
+      fail:
+        msg: mandatory test_command variable is undefined
+      when: test_command is undefined
+
     - name: Convert test_command to list
       set_fact:
         test_commands: ['{{ test_command }}']

zuul.d/general-jobs.yaml

@@ -40,8 +40,6 @@
 
          Path to operate in.
     run: playbooks/run-test-command/run.yaml
-    vars:
-      test_command: "exit 1"
 
 - job:
     name: upload-git-mirror