patch_tester.sh (nopristinecache): New shell var, set according to presence or absence of new -nopristinecache option.

	* patch_tester.sh (nopristinecache): New shell var, set according
	to presence or absence of new -nopristinecache option.
	(usage): Document new option.
	(bootntest_pristine): Implement it.

From-SVN: r169273