test_bt.sh: Check stderr for backtrace as well as stdout.

addresssanitizer likes to put backtraces on stderr.

changes/bug18934

@@ -0,0 +1,3 @@
+  o Minor features (testing):
+    - Let backtrace tests work correctly under AddressSanitizer.
+      Fixes part of bug 18934.

src/test/test_bt.sh

@@ -4,7 +4,7 @@
 exitcode=0
 
 "${builddir:-.}/src/test/test-bt-cl" backtraces || exit $?
-"${builddir:-.}/src/test/test-bt-cl" assert | "${PYTHON:-python}" "${abs_top_srcdir:-.}/src/test/bt_test.py" || exitcode="$?"
-"${builddir:-.}/src/test/test-bt-cl" crash  | "${PYTHON:-python}" "${abs_top_srcdir:-.}/src/test/bt_test.py" || exitcode="$?"
+"${builddir:-.}/src/test/test-bt-cl" assert 2>&1 | "${PYTHON:-python}" "${abs_top_srcdir:-.}/src/test/bt_test.py" || exitcode="$?"
+"${builddir:-.}/src/test/test-bt-cl" crash  2>&1 | "${PYTHON:-python}" "${abs_top_srcdir:-.}/src/test/bt_test.py" || exitcode="$?"
 
 exit ${exitcode}