ktest: Add helper function to avoid duplicate code

Several places had the following code:

    get_grub_index;
    get_version;
    install;

    start_monitor;
    return monitor;

Creating a function "start_monitor_and_boot()" replaces these mulitple
uses with a single call.

Signed-off-by: Steven Rostedt <rostedt@goodmis.org>