test/pylib: scylla_cluster: mark cluster as dirty if it fails to boot

If a cluster fails to boot, it saves the exception in
`self.start_exception` variable; the exception will be rethrown when
a test tries to start using this cluster. As explained in `before_test`:
```
    def before_test(self, name) -> None:
        """Check that  the cluster is ready for a test. If
        there was a start error, throw it here - the server is
        running when it's added to the pool, which can't be attributed
        to any specific test, throwing it here would stop a specific
        test."""
```
It's arguable whether we should blame some random test for a failure
that it didn't cause, but nevertheless, there's a problem here: the
`start_exception` will be rethrown and the test will fail, but then the
cluster will be simply returned to the pool and the next test will
attempt to use it... and so on.

Prevent this by marking the cluster as dirty the first time we rethrow
the exception.

Closes #12560

test/pylib/scylla_cluster.py

@@ -709,6 +709,8 @@ class ScyllaCluster:
         to any specific test, throwing it here would stop a specific
         test."""
         if self.start_exception:
+            # Mark as dirty so further test cases don't try to reuse this cluster.
+            self.is_dirty = True
             raise self.start_exception
 
         for server in self.running.values():