1. A method comprising: accessing, by a device, a test case that tests one or more user interfaces associated with another device, wherein the test case is defined in a spreadsheet; interpreting, by the device, the test case, wherein a row in the spreadsheet is interpreted as a test step of the test case; executing, by the device, the test case with respect to the one or more user interfaces, wherein the executing comprises: recognizing a syntax of a custom variable included in the spreadsheet; accessing a custom variable file in response to the recognizing, wherein the custom variable file stores a custom variable value; retrieving the custom variable value; and executing the test case using the custom variable value; and outputting, by the device, a test result.