oeqa/utils/command: Improve stdin handling in runCmd

Occasionally we've been seeing leftover threads from runCmd. The stdin test
assumes we clean up all threads but the code assumes that the daemonic thread
can be left behind.

The issue can be reproduced by adding a time.sleep(10) to the end of
writeThread() which will mean it stays resident past the end of the command.

We may as well add it to the threads list and clean it up properly,
hopefully removing the race in the tests from the autobuilder.

[YOCTO #13055]

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>