[gdb/testsuite] Add PR26286 kfail in gdb.threads/attach-many-short-lived-threads.exp

[gdb/testsuite] Add PR26286 kfail in gdb.threads/attach-many-short-lived-threads.exp

When running test-case gdb.threads/attach-many-short-lived-threads.exp, I run
regularly into PR26286:
...
(gdb) continue^M
Continuing.^M
[LWP ... exited]^M
  ...
[LWP ... exited]^M
^M
Program terminated with signal SIGTRAP, Trace/breakpoint trap.^M
The program no longer exists.^M
(gdb) FAIL: gdb.threads/attach-many-short-lived-threads.exp: iter 9: \
  break at break_fn: 1
...

Add a kfail for this, such that we have:
...
(gdb) KFAIL: gdb.threads/attach-many-short-lived-threads.exp: iter 9: \
  break at break_fn: 1 (PRMS: threads/26286)
...

Reviewed-By: Thiago Jung Bauermann <thiago.bauermann@linaro.org>
Tested on x86_64-linux.