Fixes the following compiler warning:

```
Compiler run successful with warnings:
Warning (2018): Function state mutability can be restricted to view
  --> test/periphery/drippie/dripchecks/CheckBalanceLow.t.sol:20:5:
   |
20 |     function test_name_succeeds() external {
   |     ^ (Relevant source part starts here and spans across multiple lines).

Warning (2018): Function state mutability can be restricted to view
  --> test/periphery/drippie/dripchecks/CheckGelatoLow.t.sol:45:5:
   |
45 |     function test_name_succeeds() external {
   |     ^ (Relevant source part starts here and spans across multiple lines).

Warning (2018): Function state mutability can be restricted to view
  --> test/periphery/drippie/dripchecks/CheckSecrets.t.sol:30:5:
   |
30 |     function test_name_succeeds() external {
   |     ^ (Relevant source part starts here and spans across multiple lines).

Warning (2018): Function state mutability can be restricted to view
  --> test/periphery/drippie/dripchecks/CheckTrue.t.sol:19:5:
   |
19 |     function test_name_succeeds() external {
   |     ^ (Relevant source part starts here and spans across multiple lines).
```

This should be caught in CI, not sure why it wasn't