Make tests fail on compilation warnings

[josojo]

We merged the following warning:

Warning (2519): This declaration shadows an existing declaration.
  --> test/ForkableBridge.t.sol:93:9:
   |
93 |         address forkmanager = forkableBridge.forkmanager();
   |         ^^^^^^^^^^^^^^^^^^^
Note: The shadowed declaration is here:
  --> test/ForkableBridge.t.sol:22:5:
   |
22 |     address public forkmanager = address(0x123);