I have a jUnit test (lets call it T1) in which I use an assertion. For that assertion to be of any value I need the validator for that assertion to be correct.
For the validator I have a test (T2) in another test Suite. Can I somehow make T2 a precondition for T1.
If that is not possible, would it be possible if T1 and T2 were in the same suite?
You think about skipping unit tests when a precondition isn't occur. Skipping unit tests by coded logic is dangerous because you won't see if there is any problem and your code even don't get test coverage. It's better to fail the more tests when a fundamental piece is broken, than to hide errors. Note that we are talking about unit tests, which should be independent and test the small pieces of software.
You can use an etalon validator which isn't part of the production code, but must be a stone simple test utility which doesn't prevent tests to run for sure. The worst thing that there could be another errors in your code, not connected to the validator, which can be delayed to emerge, just because the test code skips the critical code parts due to a precondition.
If you want to do integration or functional testing in real, you should consider using even another test framework than JUnit. In integration testing, sometimes it has sense to skip the rest of the tests and stop abusing the integration environment for minutes/hours after an unequivocally screwed beginning, but in unit testing the main point is that every piece of the code must get control.
Of course you can hack preconditions in JUnit too, for example make an asserion in the @BeforeClass/setUp fixture or putting 'if' forks into the tests, but they will fool you.