add UT for seata-spring-boot-starter module

[slievrly]

https://app.codecov.io/gh/apache/incubator-seata/tree/2.x/seata-spring-boot-starter%2Fsrc%2Fmain%2Fjava%2Forg%2Fapache%2Fseata%2Fspring%2Fboot%2Fautoconfigure

[OmCheeLin]

Please assign it to me，I’d like to work on this.