ktest: Add PRE/POST_KTEST and TEST options

In order to let the user add commands before and after ktest runs, the
PRE_KTEST and POST_KTEST options are defined. They hold shell commands
that will execute befor ktest runs its first test, as well as when it
completed its last test.

The PRE_TEST and POST_TEST will be run befor and after (respectively)
for a given test. They can either be global (done for all tests) or
defined by a single test.

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