When the data for a project is no longer needed, you can choose to delete the data from your ALM project. If you have the appropriate user permissions to clear the project history, you can choose to either clear all of the history data, or you can select to delete the history of a specific entity or field. When the history is cleared, the data displayed in the History tab of the corresponding modules within ALM is deleted. In order to clear the history, you will need to access the Clear History dialog window. In ALM versions 11.x, you can access the Clear History dialog by clicking Tools in the menu bar and selecting the option Clear History.... In ALM versions 12.x, you can access the Clear History dialog by clicking the gear icon located to the left of the Logout button in the top right corner. Once you have accessed the Clear History dialog window, you can choose to delete history data up to and including a specific date for selected Entities and Fields.

What exactly do you mean by ''test history''?

There is a function called Purge Runs that will delete the Run information from Test Instances in the Test Lab, while leaving in place the test instances and test sets in the Lab, and the test cases in the Plan, and the defects. More information is available in the product's User Guide.