Set transaction group interval to ZERO by default in tests

We were seeing non-deterministic behavior in randomized tests when
generating backtraces took enough time to cause transactions to group
in some cases, but not group in others.

Tests will need to explicitly opt into grouping if they want it by
setting the interval explicitly. We have tests in the text module that
currently test the history grouping explicitly, but I'm not sure
it's needed elsewhere.

crates/text/src/text.rs

@@ -115,6 +115,10 @@ impl History {
             undo_stack: Vec::new(),
             redo_stack: Vec::new(),
             transaction_depth: 0,
+            // Don't group transactions in tests unless we opt in, because it's a footgun.
+            #[cfg(any(test, feature = "test-support"))]
+            group_interval: Duration::ZERO,
+            #[cfg(not(any(test, feature = "test-support")))]
             group_interval: Duration::from_millis(300),
         }
     }