Encapsulate mod.rs::validate_owner checks (#68)

This PR consolidates the code blocks in
`p-token/src/entrypoint-runtime-verification.rs` checking the validity
of owner signatures as checked by `mod.rs::validate_owner`. In
particular:

- Defines `inner_test_validate_owner` which contains the logic that was
being checked in several tests.
- Replace the pertinent code blocs across the test suite with pertinent
calls to `inner_test_validate_owner`.
- Change the `multisig_is_initialised : Result<bool, ProgramError>` to
`maybe_multisig_is_initialised : Option<Result<bool, ProgramError>>` so
that `inner_test_validate_owner` doesn't depend on the `multisig` config
flag.

Observation: `inner_test_validate_owner` has the `#[inline(always)]`
attribute. It is unclear to me if that should be the case.

---------

Co-authored-by: Daniel Cumming <124537596+dkcumming@users.noreply.github.com>

Author: JuanCoRo