When people say that old engines were more reliable than modern builds, they are not making groundless statements. As technical regulations become more stringent, combustion engines are becoming more complex. Many manufacturers also now outsource the product of various components which reduces oversight of quality and precision.
The reduced oversight increases the likelihood of problems developing, making testing even more important. The introduction of various complex technology in engines means that conventional testing methods during assembly are no longer sufficient for quality assurance. There are problems that only surface when the engine is running. Manufacturers have to run the engine at the end of the line to check for potential issues. This is referred to as hot testing, in which fuel and air is supplied to the engine and actual combustion happens so problems that can happen in the real world are detected and dealt with. The problem with hot testing is that it adversely affects efficiency. After testing, the engine must be allowed to cool before the next process.
Cold testing sounds like the engine is being tested as to how it handles very low temperatures with the use of climate-controlled chamber that uses low temperature testing fluid and all that stuff. It is not. Cold testing of the engine refers to a method of testing in which the crankshaft of the engine is rotate using an electric motor. This means that no combustion happens within the engine. But it works just as well as hot testing for inspecting timing, fitment, and other mechanical issues. Because there is no combustion, the engine does not get hot. This allows the manufacturer to maximize efficiency because there is no need to wait for the engine to cool down.