Move the main Run functions into the cmd package

[dnephin]

This allows someone to run gotestsum as a library, by passing in a command line.

For example, a project could add a file test/main.go with contents:

package main

import (
	"fmt"
	"os"

	"gotest.tools/gotestsum/cmd"
)

func main() {
	name, args := os.Args[0], os.Args[1:]
	args = append([]string{"-f=testname", "--"}, args...)
	if err := cmd.Run(name, args); err != nil {
		fmt.Fprintln(os.Stderr, "ERROR ", err.Error())
		os.Exit(1)
	}
}

And run their tests with go run ./test/, which would run gotestsum without having to install the binary.

Ref: golang/go#41878