Add more tests for RefId and fix ESM3ExteriorCellRefId

elsidrequested to merge
elsid/openmw:ref_id_tests into master

Add more tests to detect changes in RefId implementation so something breaks when it happens to force developers to investigate, add more tests and don't forget to increment save game format version.

ESM3ExteriorCellRefId text serialization does not handle negative values properly. It produces unprintable charactes. Fix this and add tests.

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