Under Texas law, an employee has no right to see the contents of their employee file unless the employer voluntarily allows them to see its contents.
If an employer is performing random drug tests of employees, one would expect that they would keep records of the result. As for whether those records would be kept in the employee's file or in a separate file, that would depend upon the employer's policies, and perhaps also any law or regulation (if any) that mandates the testing.

