ktest: Add STOP_TEST_AFTER to stop the test after a period of time

Currently, if a test causes constant output but never reaches a
boot prompt, or crashes, the test will never stop. Add STOP_TEST_AFTER
to create a variable that will stop (and fail) the test after it has run
for this amount of time. The default is 10 minutes. Setting this
variable to -1 will disable it.

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