chore: update returned line number in test (#9840)

## Description

Since #8549 frames without a line number report the line number as `-1`.
This PR updates one test so it checks the correct value.

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## Reviewer Checklist
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[release branch maintenance
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Signed-off-by: Juanjo Alvarez <[email protected]>

tests/appsec/iast_memcheck/test_iast_mem_check.py

@@ -188,5 +188,4 @@ def test_stacktrace_memory_random_string_check():
 
         file_name, line_number = frame_info
         assert file_name == ""
-        # FIXME: https://datadoghq.atlassian.net/browse/APPSEC-54114
-        # assert line_number == 0
+        assert line_number == -1