sstable_tests: commonize code to compare two components.
sstable_tests: commonize code to compare two components.

The current codes assumes a particular dir/generation pair. We
will use it for a more generic case. This code could really use some
clean up, by the way. We should do it later.

Signed-off-by: Glauber Costa <f8b79557d19b40666ebe336545a0b6178af65685@scylladb.com>

kjniemi/scylla,raphaelsc/scylla,scylladb/scylla,avikivity/scylla,raphaelsc/scylla,kjniemi/scylla,duarten/scylla,scylladb/scylla,avikivity/scylla,duarten/scylla,raphaelsc/scylla,scylladb/scylla,scylladb/scylla,kjniemi/scylla,avikivity/scylla,duarten/scylla