Fix incorrect core file in run_tests.sh

[chenBright]

What problem does this PR solve?

Issue Number:

brpc/test/run_tests.sh

Lines 37 to 44 in 6a335aa

	print_bt () {
	# find newest core file
	COREFILE=$(find . -name "core*" -type f -printf "%T@ %p\n" | sort -k 1 -n | cut -d' ' -f 2- | tail -n 1)
	if [ ! -z "$COREFILE" ]; then
	>&2 echo "corefile=$COREFILE prog=$1"
	gdb -c "$COREFILE" $1 -ex "thread apply all bt" -ex "set pagination 0" -batch;
	fi
	}

run_tests.sh中有问题，认为最新的core文件一定是失败的ut生成的，但是ci中出现了对不上的情况：

1 FAILED TEST
corefile=./core.test_butil.8086 prog=brpc_grpc_protocol_unittest

Problem Summary:

What is changed and the side effects?

Changed:

先将旧的core文件删除。

Side effects:

• Performance effects(性能影响):

• Breaking backward compatibility(向后兼容性):

---

Check List:

• Please make sure your changes are compilable(请确保你的更改可以通过编译).
• When providing us with a new feature, it is best to add related tests(如果你向我们增加一个新的功能, 请添加相关测试).
• Please follow Contributor Covenant Code of Conduct.(请遵循贡献者准则).