Add more string tests for IndexOf and utf-8

[ericoporto]

IndexOf could use a few additional utf-8 tests to ensure we are correctly getting the right index when either or both the string and needle have utf-8 characters.

There are probably other utf-8 tests that would be useful to add to strings but I don't remember right now any other cases.

[ivan-mogilko]

Utf-8 specifics are affecting any method that:

• manipulates individual character
• involves a character index or length of string.

for AGS 3.*:

• AppendChar - because it must convert unicode index to utf-8 representation
• CompareTo - in case-insensitive case
• EndsWith - in case-insensitive case
• IndexOf - because it appears to be case-insensitive and uses "lowercase" for converting characters before comparison
• LowerCase - converts characters
• Replace - in case-insensitive case
• ReplaceCharAt
• StartsWith - in case-insensitive case
• Substring
• Truncate
• UpperCase - converts characters
• Chars[] property
• Length property

for AGS 4.*:

• IndexOf - after adding begin,end parameters
• Trim - in case of extra unicode whitespace characters

[ericoporto]

ok, I added a few tests, but I also found a small bug in the new compiler.

https://github.com/adventuregamestudio/ags-test-games/blob/7d909cb13c977af39605069c2f80ea33df8ddc44/ags4/auto-test/test-string.asc#L16-L20

This gives the error "test-string.asc(19): Expected a quote mark but found '¤' instead".

[ericoporto]

I think some string UpperCase/LowerCase tests would be useful to have by just having a few strings with characters from different alphabets - like lower/upper of Greek, and so on.

[ivan-mogilko]

This gives the error "test-string.asc(19): Expected a quote mark but found '¤' instead".

I cannot remember what happened when I tested unicode char literals. But they are supposed to work, at least with the new compiler which is more advanced. Could you open a bug ticket in the main repository?

[ericoporto]

Sure, I will create a proper bug report later. To be specific this is only a bug in the new compiler, it works fine in the old one.