ktest.pl: Add a NOT operator

There is a NOT DEFINED operator, but there is not an operator that can
negate any other expression.

 For example: NOT (${FOO} == boot || ${BAR} == run)

Add the keyword NOT to allow the ktest.pl config files to negate operators.

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