test: Assert traces_sampler called once (#4450)

Currently, this test only asserts that the `traces_sampler` is called at
least once with the given arguments. However, we would expect it to be
called exactly once.

This PR changes the assertion to assert that the `traces_sampler` was
called exactly one time, and that that call included the given
arguments.

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Author: szokeasaurusrex

tests/tracing/test_sampling.py

@@ -218,7 +218,7 @@ def test_passes_attributes_from_start_span_to_traces_sampler(
     sentry_init(traces_sampler=traces_sampler)
 
     with start_span(attributes={"dogs": "yes", "cats": "maybe"}):
-        traces_sampler.assert_any_call(
+        traces_sampler.assert_called_once_with(
             DictionaryContaining({"dogs": "yes", "cats": "maybe"})
         )