lib/mnit: globally protect from NIT_TESTING

Signed-off-by: Jean Privat <jean@pryen.org>
lib/string_exp: add a missing FFI `import UnicodeChar.len`

Signed-off-by: Jean Privat <jean@pryen.org>
tests: add java_io to cc.skip

See #696

Signed-off-by: Jean Privat <jean@pryen.org>