Flaky test, 3456 can appear in timestamp

This `assert` is flaky as the string `3456` can appear in the timestamp.

https://github.com/equinor/res2df/blob/3a25347d2acfa7e96f672678d2ff91f6d5bc4076/tests/test_common.py#L206

Observed in real life:
```
> assert string.find("3456") > string.find("1234")
E AssertionError: assert 78 > 139
E + where 78 = ('3456')
E + where = '-- Output file printed by tests.test_common 1.3.14\n-- at 2026-06-05 14:25:42.345613\n\nEQUIL\n-- Z PRESSURE OWC PCOWC GOC\n 2469.0 1234 100.0 0.0 0.0 /\n 2469.0 3456 100.0 0.0 0.0 /\n'.find
E + and 139 = ('1234')
E + where = '-- Output file printed by tests.test_common 1.3.14\n-- at 2026-06-05 14:25:42.345613\n\nEQUIL\n-- Z PRESSURE OWC PCOWC GOC\n 2469.0 1234 100.0 0.0 0.0 /\n 2469.0 3456 100.0 0.0 0.0 /\n'.find
```