tests: net: lwm2m: Add tests for string truncation

lwm2m_get_string() and lwm2m_set_string() API does not allow
strings to be truncated and ensures NUL termination
regardles of using string or opaque resources to store the string.

Add tests to verify that is also true on multi-byte UTF-8
strings.

Signed-off-by: Seppo Takalo <seppo.takalo@nordicsemi.no>