LisaTest: Add permitted_fail_pct attribute
LisaTest: Add permitted_fail_pct attribute

This allows specifying a percentage of iterations of a given
test (tuple wload, conf, test method) can be permitted to fail.

If some tests fail but not enough to cross this percentage threshold, a warning
is printed.

arnoldlu/lisa,ARM-software/lisa,mdigiorgio/lisa,credp/lisa,credp/lisa,mdigiorgio/lisa,bjackman/lisa,credp/lisa,ARM-software/lisa,ARM-software/lisa,ARM-software/lisa,bjackman/lisa,arnoldlu/lisa,credp/lisa