gdb/testsuite: use 'return' in gdb_test_no_output

gdb/testsuite: use 'return' in gdb_test_no_output

A TCL proc will return the return value of the last command executed
within the proc's body if there is no explicit return call, so
gdb_test_no_output is already returning the return value of
gdb_test_multiple.

However, I'm not a fan of (relying on) this implicit return value
behaviour -- I prefer to be explicit about what we are doing.  So in
this commit I have extended the comment on gdb_test_no_output to
document the possible return values (just as gdb_test does), and
explicitly call return.

This should make no different to our testing, but I think it's clearer
now what the gdb_test_no_output proc is expected to do.

The two tests gdb.base/auxv.exp and gdb.base/list.exp both rely on the
return value of gdb_test_no_output, and continue to pass after this
change.

I also spotted that gdb.base/watchpoint.exp could be updated to make
use of gdb_test_no_output, so I did that.

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