Remove abort handler from the backtrace generator

The abort handler masks the exit status of the backtrace generator by
capturing the abort signal from the backtrace handler and exiting with
zero. Because the output of the backtrace generator is meant to be piped
to `bt_test.py`, its exit status is unimportant and is currently
ignored.

The abort handler calls `exit(3)` which is not asynchronous-signal-safe
and calling it in this context is undefined behavior [0].

Closes ticket 21026.

[0] https://www.securecoding.cert.org/confluence/x/34At

changes/ticket21026

@@ -0,0 +1,3 @@
+  o Minor bugfixes (testing):
+    - Remove undefined behavior from the backtrace generator by removing
+      its signal handler. Fixes bug 21026; bugfix on 0.2.5.2-alpha.

src/test/test_bt_cl.c

@@ -19,14 +19,12 @@ static int crashtype = 0;
 
 #ifdef __GNUC__
 #define NOINLINE __attribute__((noinline))
-#define NORETURN __attribute__((noreturn))
 #endif
 
 int crash(int x) NOINLINE;
 int oh_what(int x) NOINLINE;
 int a_tangled_web(int x) NOINLINE;
 int we_weave(int x) NOINLINE;
-static void abort_handler(int s) NORETURN;
 
 #ifdef HAVE_CFLAG_WNULL_DEREFERENCE
 DISABLE_GCC_WARNING(null-dereference)
@@ -76,13 +74,6 @@ we_weave(int x)
   return a_tangled_web(x) + a_tangled_web(x+1);
 }
 
-static void
-abort_handler(int s)
-{
-  (void)s;
-  exit(0);
-}
-
 int
 main(int argc, char **argv)
 {
@@ -120,8 +111,6 @@ main(int argc, char **argv)
 
   configure_backtrace_handler(NULL);
 
-  signal(SIGABRT, abort_handler);
-
   printf("%d\n", we_weave(2));
 
   clean_up_backtrace_handler();