Add flag to prefix testsuite name

[bbrks]

I would like the ability to prefix the testsuite name with a custom string.

This will allow me to run tests, and generate unique XML reports for various combinations of $GOOS, $GOARCH, and other custom build tags.

E.g: -package-prefix="linux/arm/"

<testsuite ... name="linux/arm/github.com/example/example">

This is mainly so that I can have Jenkins read multiple junit reports, and aggregate them in a single pipeline job:

Would you be open to a PR that adds a flag that will prepend something to the test suite package name like above?