Fix test warning regexp after warning message was reworded

The warning text changed from "All observed delays are identical"
to "Near-zero variance in observed delays" but the test regexp
was not updated, causing a case-sensitive mismatch.

Co-Authored-By: Sam Abbott <s.e.abbott12@gmail.com>

Author: seabbs-bot

tests/testthat/test-estimate_dist.R

@@ -465,7 +465,7 @@ test_that("warns when all observed delays are identical", {
       )),
       error = function(e) NULL
     ),
-    "All observed delays are identical" # matches cli_warn text
+    "Near-zero variance in observed delays" # matches cli_warn text
   )
 })