Initialise worker pods.

[sinistral]

The initial pod ID and shutdown hooks are initialised by Boot when the app is
launched, but not again when pod pools are spun up. Boot's fileset and tmpfile
machinery hooks into the pod lifecycle depends on pods being propertly
initialised; the following errors were encounted while testing a task that
modifies the Boot fileset.

  actual: java.lang.NullPointerException: null
 at clojure.lang.RT.longCast (RT.java:1241)
    boot.tmpregistry$dir_id.invoke (tmpregistry.clj:31)
    boot.tmpregistry$registry.invoke (tmpregistry.clj:93)
    boot.core/fn (core.clj:44)
    clojure.lang.Delay.deref (Delay.java:37)
    clojure.core$deref.invokeStatic (core.clj:2228)
    clojure.core$deref.invoke (core.clj:2214)
    boot.core$tmp_dir_STAR_.invoke (core.clj:98)
    boot.core$tmp_dir_STAR__STAR_.doInvoke (core.clj:111)
    clojure.lang.RestFn.invoke (RestFn.java:423)
    boot.core$tmp_dir_BANG_.invoke (core.clj:356)

  actual: java.lang.NullPointerException: null
 at clojure.lang.Reflector.invokeInstanceMethod (Reflector.java:26)
    boot.pod$add_shutdown_hook_BANG_.invoke (pod.clj:345)
    boot.tmpregistry.TmpRegistry._init_BANG_ (tmpregistry.clj:80)
    boot.tmpregistry$init_BANG_.invoke (tmpregistry.clj:55)
    boot.core/fn (core.clj:44)
    clojure.lang.Delay.deref (Delay.java:37)
    clojure.core$deref.invokeStatic (core.clj:2228)
    clojure.core$deref.invoke (core.clj:2214)
    boot.core$tmp_dir_STAR_.invoke (core.clj:98)
    boot.core$tmp_dir_STAR__STAR_.doInvoke (core.clj:111)
    clojure.lang.RestFn.invoke (RestFn.java:423)
    boot.core$tmp_dir_BANG_.invoke (core.clj:356)

See:

• https://github.com/boot-clj/boot/blob/dce24183711747fc43b6e13c061cd5cb073b4e94/boot/base/src/main/java/boot/App.java#L407
• https://github.com/boot-clj/boot/blob/dce24183711747fc43b6e13c061cd5cb073b4e94/boot/core/src/boot/tmpregistry.clj#L31

I may be setting the fileset up incorrectly in my test, and if there's a known better way to do this, I'd appreciate any guidance.