Tests for UTF-8 multi-byte characters and other corner cases

[msgilligan]

It would be nice to have unit tests for OmniJ itself and regtest integration tests for Omni Core.

[dexX7]

We had a bit of trouble with non-UTF8 multibyte strings in Omni Core (see OmniLayer/omnicore#108), and I ended up with sanitizing RPC results.

OmniLayer/omnicore#177 added locale-independent functions to validate and sanitize UTF-8 encoded strings, and it has a bunch of unit tests.

edit: see also my note in OmniLayer/omnicore#177 (comment):

While the validation should be able to handle any UTF-8 encoded string, including foreign character sets, it is unfortunally partially overruled by the sanitization of json_spirit::, which is not locale-independent, and neither supports multibyte character sequences.

In practise this means this PR ensures that no invalid characters slip through (as outlined in 108), but it doesn't imply that Omni Core can display all kinds of characters, which are considered as properly encoded.

[ahmedrem55538]

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[ahmedrem55538]

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