Merge branch 'cp/unit-test-reftable-merged'

Merge branch 'cp/unit-test-reftable-merged'

Another reftable test has been ported to use the unit test framework.

* cp/unit-test-reftable-merged:
  t-reftable-merged: add test for REFTABLE_FORMAT_ERROR
  t-reftable-merged: use reftable_ref_record_equal to compare ref records
  t-reftable-merged: add tests for reftable_merged_table_max_update_index
  t-reftable-merged: improve the const-correctness of helper functions
  t-reftable-merged: improve the test t_merged_single_record()
  t: harmonize t-reftable-merged.c with coding guidelines
  t: move reftable/merged_test.c to the unit testing framework