Improve option support & tests for key-value trees

Improve option support & tests for key-value trees

Add basic tests for the behavior of different option types with
key-value tree handling.  For now, the test covers only the "adjust"
function.  Make enum options serialize as strings instead of integers,
which improves behavior of gmx dump and gmx check, and can also be more
robust for backward/forward compatibility.

Improve the way the key-value tree serialization tests are done: use the
in-memory serializer to test roundtrips, instead of relying on the
reference data.  There is less code this way, and the tests behave
better in the absence of reference data.

Change-Id: I5e52265333f397bd6e058636691cbfd21092df0d