testAllFilesInSysAreNotWritable: Add trace_marker exception

Add an exception for a world writable
/sys/kernel/debug/tracing/trace_marker . This file is used as part
of the systrace functionality on user builds, and allows application
developers to trace system performance for their application.

Tracing data is only accessible to the shell user, but any user
can write to the trace_marker file.

Bug: 6292683
Bug: 6513400
Change-Id: Ia00540652b137f4d8f5003e7c3d5120732258532