fmp4mux: Test `test_buffer_flags_multi_stream` is racy
This especially seems to happen on Windows, but not only.
test test_buffer_flags_multi_stream has been running for over 60 seconds
test test_buffer_flags_multi_stream ... FAILED
failures:
---- test_buffer_flags_multi_stream stdout ----
thread 'test_buffer_flags_multi_stream' panicked at 'called `Result::unwrap()` on an `Err` value: BoolError { message: "Failed to pull buffer", filename: "C:\\Users\\ContainerAdministrator\\.cargo\\git\\checkouts\\gstreamer-rs-79e52a2d27eb91a3\\da84e72\\gstreamer-check\\src\\harness.rs", function: "gstreamer_check::harness", line: 267 }', mux\fmp4\tests\tests.rs:315:28
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
failures:
test_buffer_flags_multi_stream
test result: FAILED. 8 passed; 1 failed; 0 ignored; 0 measured; 0 filtered out; finished in 60.07s
error: test failed, to rerun pass `-p gst-plugin-fmp4 --test tests`