fix up tests

This fixes up some git and oscommand tests, and pulls some tests into commit_list_builder_test.go

I've also made the NewDummyBlah functions public so that I didn't need to duplicate them across packages
I've also given OSCommand a SetCommand() method for setting the command on the struct
I've also created a file utils.go in the test package for creating convient 'CommandSwapper's, which
basically enable you to assert a sequence of commands on the command line, and swap each one out for
a different one to actually be executed

pkg/commands/os.go

@@ -50,6 +50,12 @@ func NewOSCommand(log *logrus.Entry, config config.AppConfigurer) *OSCommand {
 	}
 }
 
+// SetCommand sets the command function used by the struct.
+// To be used for testing only
+func (c *OSCommand) SetCommand(cmd func(string, ...string) *exec.Cmd) {
+	c.command = cmd
+}
+
 // RunCommandWithOutput wrapper around commands returning their output and error
 func (c *OSCommand) RunCommandWithOutput(command string) (string, error) {
 	c.Log.WithField("command", command).Info("RunCommand")