GDB: testsuite: threads: Don't return 0 from top-level (sed)

This patch is a bit different than the others.  I noticed that
gdb.threads has many cases of "return 0" from the top level, so this
patch is purely the result of running:

$ sed -i 's/^    return 0/    return/' *.exp*

inside gdb/testsuite/gdb.threads.

There were a number of false positives so not all sed changes were
incorporated.

Approved-By: Tom Tromey <tom@tromey.com>