In practice, States must all implement the full interface, so checking
for each method set only leaves gaps where tests could be skipped.
Change the helper to only accept a full state.State implementation.
Add some Lineage, Version, and TFVersion checks to TestState to avoid
regressions.
Compare the copy test against the immediate State returnedm rather than
our previous "current" state.
Check that the states round-trip and still marhsal identically via
MarshalEqual.
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