[PATCH] gdb.python/py-evthreads.exp: fix racy test (PR testsuite/12649)

[Joel Brobecker]

> It is a difference as it requires the end of the read buffer that way.
> 
> If GDB outputs some
> (gdb) unexpected_result
> 
> then (without any read1 hacks) -re "$gdb_prompt $" will FAIL/timeout as it
> should while -re "$gdb_prompt " could give a false PASS.
> 
> (I do not think it requires a new commit, it is up to you.)

Let's fix it. If anything, we should be consistent. I might have
a minute later today, but I don't want to make the change without
running the testcase... So, I won't mind if someone gets to it before
I do.

-- 
Joel