tests: lwm2m: Fix too small test buffer

ZTEST(lwm2m_registry, test_strings) is using opaque resource
0/0/3 to write a test string "coap://127.0.0.1" which
happens to be same length as default key size.
Need more room to add end marker and verify it.

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