Add a test for JavaScript UTF-8 <=> UTF-16 conversion

JavaScript uses UTF-16 but FlatBuffers uses UTF-8. This commit tests the code
that does the conversion between the two encodings. The last entry in the array
is tricky because each code point actually requires two UTF-16 code units,
unlike the other examples. The current JSON output of flatc actually handles
this case incorrectly (it generates invalid JSON with UTF-8 code units). The
generated JavaScript code passes these tests fine, however.