For anyone who comes across this, the file name should correspond to how you import the library. In my case, this seems to work regardless of the testfoo.d.ts file being located in the types directory:

// testfoo.d.ts   
declare module "@/public/assets/scripts/testfoo" {
  export interface ITest {
    disabled: boolean;
  }

  export class TestCl {
    constructor();
    go(): void;
  }
}