tests: remove redundant Python tests that are already covered by existing Go tests

Multiple python tests from filebeat/tests/system have Go equivalents in the tree, for example filebeat/tests/system/test_multiline.py and TestParsersMultilineMaxLines, TestParsersMultilineTimeout and friends are defined in filebeat/input/filestream/parsers_integration_test.go.

We need to investigate and confirm which python tests already exist in Go, write the missing ones using Go, and finally delete the python test file.

These python tests started flaking more recently, so we need to take measures to confirm the equivalent Go tests are stable with -count.

Compare the Python tests in filebeat/tests/system/test_multiline.py with the Go coverage in filebeat/input/filestream/parsers_integration_test.go, including TestParsersMultilineMaxLines and TestParsersMultilineTimeout. Run the equivalent Go tests with -count to check stability, identify missing coverage, and confirm the Python file can be removed once all relevant cases are covered.

Written by the indexing model from the issue text.