tests: net: lib: lwm2m: interop: Fix strip-with-multi-characters (B005)

All strip functions remove any of the provided characters instead of a
prefix/suffix. This is likely a bug.

See https://docs.astral.sh/ruff/rules/strip-with-multi-characters/



Signed-off-by: Pieter De Gendt <pieter.degendt@basalte.be>