Share code in encoding conversion tests

intl/uconv/tests/unit/CharsetConversionTests.js

intl/uconv/tests/unit/test_bug116882.js

intl/uconv/tests/unit/test_bug367026.js

intl/uconv/tests/unit/test_bug396637.js

intl/uconv/tests/unit/test_bug399257.js

intl/uconv/tests/unit/test_bug457886.js

intl/uconv/tests/unit/test_decode_8859-1.js

intl/uconv/tests/unit/test_decode_8859-10.js

intl/uconv/tests/unit/test_decode_8859-11.js

intl/uconv/tests/unit/test_decode_8859-13.js

intl/uconv/tests/unit/test_decode_8859-14.js

intl/uconv/tests/unit/test_decode_8859-15.js

intl/uconv/tests/unit/test_decode_8859-2.js

intl/uconv/tests/unit/test_decode_8859-3.js

intl/uconv/tests/unit/test_decode_8859-4.js

intl/uconv/tests/unit/test_decode_8859-5.js

intl/uconv/tests/unit/test_decode_8859-6.js

intl/uconv/tests/unit/test_decode_8859-7.js

intl/uconv/tests/unit/test_decode_8859-8.js

intl/uconv/tests/unit/test_decode_8859-9.js