Improve test coverage

[DrZoltanFazekas]

The relevant output when running forge coverage --ir-minimum:

╭---------------------------------+-------------------+-------------------+-----------------+------------------╮
| File                            | % Lines           | % Statements      | % Branches      | % Funcs          |
+==============================================================================================================+
| src/BaseDelegation.sol          | 82.89% (247/298)  | 84.23% (283/336)  | 17.72% (14/79)  | 77.50% (31/40)   |
|---------------------------------+-------------------+-------------------+-----------------+------------------|
| src/LiquidDelegation.sol        | 78.90% (86/109)   | 82.20% (97/118)   | 26.32% (5/19)   | 76.19% (16/21)   |
|---------------------------------+-------------------+-------------------+-----------------+------------------|
| src/NonLiquidDelegation.sol     | 85.28% (168/197)  | 87.08% (182/209)  | 46.88% (15/32)  | 71.43% (25/35)   |
|---------------------------------+-------------------+-------------------+-----------------+------------------|
| src/NonRebasingLST.sol          | 100.00% (4/4)     | 100.00% (2/2)     | 100.00% (0/0)   | 100.00% (2/2)    |
|---------------------------------+-------------------+-------------------+-----------------+------------------|
| src/WithdrawalQueue.sol         | 100.00% (14/14)   | 100.00% (16/16)   | 0.00% (0/2)     | 100.00% (5/5)    |
|---------------------------------+-------------------+-------------------+-----------------+------------------|
| Total                           | 66.23% (853/1288) | 67.76% (948/1399) | 21.17% (47/222) | 66.32% (126/190) |
╰---------------------------------+-------------------+-------------------+-----------------+------------------╯