tests: consider testing of integer overflow

With:

- libcon4m b4fdc8e2e70b0ef18a554562702e2db1e0c9aff0
- x86_64
- Linux 6.9.7
- gcc 14.1

Running `c4test` with this `.c4m` file:

```text
"""
Test of integer overflow.
"""
"""
$output:
9223372036854775807
-'..--).0-*(+,))+(0(
"""

x = 9223372036854775807
print(x)

x += 1
print(x)
```

currently passes. Probably not a high priority. But if we want an error at some point, and test coverage for similar things, let's track that in this issue.