Fix Frax Test inheritance for child repo

[tom2o17]

Fix relative vs strict import for use in inheriting repo.

Compiler run failed:
Error (6275): Source "src/StringsHelper.sol" not found: File not found. Searched the following locations: "/Users/tc/Documents/GitHub/demo/frax-flashloan".
ParserError: Source "src/StringsHelper.sol" not found: File not found. Searched the following locations: "/Users/tc/Documents/GitHub/demo/frax-flashloan".
 --> node_modules/frax-standard-solidity/src/FraxTest.sol:6:1:
  |
6 | import { Strings } from "src/StringsHelper.sol";
  | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

tc@TCs-MacBook-Pro frax-flashloan % 

[pegahcarter]

LGTM.