ci: make script should print ut coverage rate; 让 make 脚本打印单测覆盖率

[seeflood]

What would you like to be added:
@wenxuwan is struggling with the codecov error in #556 .
He wants to run ut and then get the coverage rate locally.
I tried to make style.coverage but didn't get the overall coverage:

I also tried make check.unit but it didn't print coverage too.

I think make script should print the overall ut coverage rate, e.g. "60%"

Why is this needed:
Make code contribution easier