testsuite: Fix PR threads/19422 regression + Guile regression

testsuite: Fix PR threads/19422 regression + Guile regression

The PR threads/19422 patchset added a new regression.

Additionally below it there was already a regression if --with-guile (which is
default if Guile is found) was used.

racy case #1:

(xgdb) PASS: gdb.gdb/selftest.exp: Set xgdb_prompt
^M
Thread 1 "xgdb" received signal SIGINT, Interrupt.^M
0x00007ffff583bfdd in poll () from /lib64/libc.so.6^M
(gdb) FAIL: gdb.gdb/selftest.exp: send ^C to child process
signal SIGINT^M
Continuing with signal SIGINT.^M
^C^M
Thread 1 "xgdb" received signal SIGINT, Interrupt.^M
0x00007ffff5779da0 in sigprocmask () from /lib64/libc.so.6^M
(gdb) PASS: gdb.gdb/selftest.exp: send SIGINT signal to child process
backtrace^M
errstring=errstring@entry=0x7e0e6c "", mask=mask@entry=RETURN_MASK_ALL) at exceptions.c:240^M
errstring=errstring@entry=0x7e0e6c "", mask=mask@entry=RETURN_MASK_ALL) at exceptions.c:240^M
(gdb) PASS: gdb.gdb/selftest.exp: backtrace through signal handler

racy case #2:

(xgdb) PASS: gdb.gdb/selftest.exp: Set xgdb_prompt
^M
Thread 1 "xgdb" received signal SIGINT, Interrupt.^M
0x00007ffff583bfdd in poll () from /lib64/libc.so.6^M
(gdb) FAIL: gdb.gdb/selftest.exp: send ^C to child process
signal SIGINT^M
Continuing with signal SIGINT.^M
^C^M
Thread 2 "xgdb" received signal SIGINT, Interrupt.^M
[Switching to Thread 0x7ffff3b7f700 (LWP 13227)]^M
0x00007ffff6b88b10 in pthread_cond_wait@@GLIBC_2.3.2 () from /lib64/libpthread.so.0^M
(gdb) PASS: gdb.gdb/selftest.exp: send SIGINT signal to child process
backtrace^M
(gdb) FAIL: gdb.gdb/selftest.exp: backtrace through signal handler

Pedro Alves:
Not all targets support thread names, and even those that do, not all
use the program name as default thread name -- I think that's only true
for GNU/Linux, actually.  So I think it's best to not expect that, like:
            -re "(Thread .*|Program) received signal SIGINT.*$gdb_prompt $" {

gdb/testsuite/ChangeLog
2016-01-22  Jan Kratochvil  <jan.kratochvil@redhat.com>

	Fix testsuite compatibility with Guile.
	* gdb.gdb/selftest.exp (send ^C to child process): Accept also Thread.
	(thread 1): New test for backtrace through signal handler.