Add helper function and some documentation to parse shorthand go test flags

[valdar]

Since pflag package ignores flags with '-test.' prefix see:
#63
#238

would be nice to add an helper function and some doc to work around that.

[Vibhuair20]

can you assign this to me so i can start working on it

[valdar]

can you assign this to me so i can start working on it

I don't have permissions to assign issues, beside this is already implemented in the linked pr.