Enable SystemUse24HourClockPolicyTest under SingleProcessMash

It passes for me locally. I don't see anything in the test or the
underlying code that depends on the mash mode. I'm not sure why it
was being skipped.

Bug: none
Change-Id: I300bd010ffa68c3cb5c6efec2df316752971084c
Reviewed-on: https://chromium-review.googlesource.com/c/1277954Reviewed-by: James Cook <jamescook@chromium.org>
Cr-Commit-Position: refs/heads/master@{#599044}

testing/buildbot/filters/chromeos.single_process_mash.browser_tests.filter

@@ -211,9 +211,6 @@
 -SigninProfileAppsPolicyPerChannelTest.*
 -SigninProfileAppsPolicyTest.*
 
-# Clock policy tests.
--SystemUse24HourClockPolicyTest.*
-
 # Unmanaged platform keys tests.
 -Unmanaged/UnmanagedPlatformKeysTest.*