Test `process.test_start_with_output_prefix_and_large_output` is flaky

[locker]

Failed tests:
-------------

1) process.test_start_with_output_prefix_and_large_output
/home/runner/work/luatest/luatest/test/process_test.lua:64: Could not find substring "test_prefix |" in string ""
stack traceback:
	...
	[C]: in function 'xpcall'
Captured stdout:
test_prefix | Hello


Ran 296 tests in 21.714 seconds, 295 succeeded, 1 failed, 9 skipped

Link to CI failure