Fix unstable test `TestLocalDirWatcher/Watch_function_should_have_the_correct_result`

[YangKeao]

Development Task

Ref https://do.pingcap.net/jenkins/blue/organizations/jenkins/pingcap%2Ftiproxy%2Fghpr_unit_test/detail/ghpr_unit_test/718/pipeline/

[YangKeao]

/assign @YangKeao