Improve test coverage

[dcposch]

Those are rookie numbers

Before

+-----------------------+------------------+------------------+----------------+----------------+
| File                  | % Lines          | % Statements     | % Branches     | % Funcs        |
+===============================================================================================+
| script/Deploy.s.sol   | 0.00% (0/7)      | 0.00% (0/6)      | 0.00% (0/2)    | 0.00% (0/1)    |
|-----------------------+------------------+------------------+----------------+----------------|
| src/BtcMirror.sol     | 94.87% (74/78)   | 96.15% (100/104) | 57.41% (31/54) | 100.00% (8/8)  |
|-----------------------+------------------+------------------+----------------+----------------|
| src/BtcProofUtils.sol | 79.55% (70/88)   | 78.10% (82/105)  | 44.12% (15/34) | 77.78% (7/9)   |
|-----------------------+------------------+------------------+----------------+----------------|
| src/BtcTxVerifier.sol | 100.00% (6/6)    | 100.00% (8/8)    | 66.67% (4/6)   | 100.00% (1/1)  |
|-----------------------+------------------+------------------+----------------+----------------|
| src/Endian.sol        | 68.42% (13/19)   | 68.42% (13/19)   | 100.00% (0/0)  | 50.00% (2/4)   |
|-----------------------+------------------+------------------+----------------+----------------|
| Total                 | 82.32% (163/198) | 83.88% (203/242) | 52.08% (50/96) | 78.26% (18/23) |
+-----------------------+------------------+------------------+----------------+----------------+

Goal

Excluding the deploy script,

• 100% function coverage
• 100% statement coverage
• 100% branch coverage