Fix Interop/SizeConst/SizeConstTest to successfully run in all locales. (#20268)

Fix Interop/SizeConst/SizeConstTest to successfully run in all locales. (#20268)

This test initially would fail when run on a multibyte character set with characters for the original value of `s.arr` such as zh-CH. We cannot change the marshaler since there is not a good way to determine how many bytes/character we need at type-load-time (when the size of the native type is determined).

This change allows us to accurately run this test on Windows machines with multi-byte character sets.

Fixes #7793.