Restore 'shell' feature in test package sandbox policy.

The 'shell' feature removal CL was reverted after the sandbox policy
files had been split into production and test versions, so the revert
did not apply to the new test process policy.

TBR: kmarshall
Bug: 823927, 836341
Change-Id: I9a4c75cb01c255887a4928c31ab199e039d53397
Reviewed-on: https://chromium-review.googlesource.com/1027441Reviewed-by: Wez <wez@chromium.org>
Commit-Queue: Wez <wez@chromium.org>
Cr-Commit-Position: refs/heads/master@{#553500}

build/config/fuchsia/testing_sandbox_policy

 {
-  "features": [ "persistent-storage", "system-temp" ],
+  "features": [ "persistent-storage", "shell", "system-temp" ],
   "dev": ["null", "zero"]
 }