Dump a backtrace with gdb

Because Ruby often fails to dump a C backtrace.

bootstraptest/runner.rb

@@ -530,6 +530,7 @@ def cleanup_coredump
     core_path = "/tmp/bootstraptest-core.#{Time.now.utc.iso8601}"
     warn "A core file is found. Saving it at: #{core_path.dump}"
     FileUtils.mv('core', core_path)
+    system('gdb', RbConfig.ruby, '-c', core_path, '-ex', 'bt', '-batch')
   end
   FileUtils.rm_f Dir.glob('core.*')
   FileUtils.rm_f @ruby+'.stackdump' if @ruby

tool/lib/test/unit.rb

@@ -357,6 +357,7 @@ module Test
           core_path = "/tmp/test-unit-core.#{Time.now.utc.iso8601}"
           warn "A core file is found. Saving it at: #{core_path.dump}"
           FileUtils.mv('core', core_path)
+          system('gdb', RbConfig.ruby, '-c', core_path, '-ex', 'bt', '-batch')
         end
         STDERR.flush
         exit c