I think this question is similar to your first one: How to Run TestNG Tests on Jenkins. have a look and let me know.
Regarding the second one it is probably technically possible, but it is never a good idea.
When you write a test you are defining the expected behavior of your code: a failing test means you found a bug!
If the 70% threshold is due to unstable tests (aka tests failing randomly) than my only advice is: live with build unstability or rewrite them (the second is way way better). What happens if the threshold "hides" a real bug?