Make `revertStrings: debug` the default in tests

Related to https://github.com/argotorg/solidity/pull/16483#discussion_r2829850997

### Description
Isoltest has a `revertStrings` setting. Make it default to `debug`.

### Motivation
The current default is the equivalent of `--revert-strings default`, which means that on validations we revert with no message (`revert(0, 0)`). This is primarily motivated by gas cost so it makes sense as a default for the compiler. However, we do not have that constraint when running tests and `--revert-strings debug` would be a more useful default.

### Example
By default expectations look like this:
```solidity
// ----
// f(uint256[]): 42 -> FAILURE
```

You can explicitly request more verbose debug strings:
```solidity
// ====
// revertStrings: debug
// ----
// f(uint256[]): 42 -> FAILURE, hex"08c379a0", 0x20, 43, "ABI decoding: invalid calldata a", "rray length"
```

It should be the default instead.
```solidity
// ----
// f(uint256[]): 42 -> FAILURE, hex"08c379a0", 0x20, 43, "ABI decoding: invalid calldata a", "rray length"
```

Start by locating isoltest's handling of the revertStrings setting and the existing tests or examples that exercise revert expectations. Change only the test default while preserving the explicit debug override, then run the relevant isoltest tests and verify the default output includes the debug revert data shown in the issue.

Written by the indexing model from the issue text.