As said like the title, this question haunted me for a long time, I would usually use the phrase experimental data, but I have ever been told experiment data has the identical meaning when referring to the data which was obtained by doing some experiments, I am not sure which one is right and Does testing data have any different meaning than experimental data？
The usual expression, if the data comes form an experiment, is experimental data. Experimental here is an adjective; as such, it has three main meanings, and the one we are interested in is "of, for, from, or related to an experiment". The others include "serving as an experiment", which can lead to confusion in the phrase experimental equipment, but we shan't worry about that now.
Experiment data makes sense, seeing experiment as an attributive noun, but I've never come across that in British English, nor American English (though I have had less exposure to that). It sounds unnatural to me. However, study data, that produced in a study (used more often in social science and medical research, in my experience), follows that pattern.
A comment has noted the alternative empirical data. In modern English, this means data that comes from real-world observations rather than that produced in a deliberate experiment. In an experiment, the experimenter controls conditions. In an empirical study, they record the conditions and the results, but do not control them. In archaic usage, experiment referred to both, and to more besides. To know something experimentally meant to know it from experience.
Finally, you ask about testing data. This is equivalent to experimental data, but produced in a test rather than an experiment. A test in this case is where you are testing some process, equipment or otherwise to make sure it behaves as expected/intended. The closely related test data is the data used in such a test, often used in software systems, such as for regression testing, making sure that a new version of software behaves the same as the old version on the same data.