Add --ref-data update-changed

Add --ref-data update-changed

Add an option to update only those reference data entries that fail the
test.  This makes it somewhat easier to maintain tests with
floating-point reference data (as long as those floating-point values
themselves do not change), since it is possible to update other parts of
the test data without a double-precision build.

User-/developer-facing documentation will be added separately together
with more comprehensive documentation of the reference data framework
later.

Change-Id: Id03346358b91c5859e8ad5614f4ff4c4691e8594