Make tests compatible with OTP 16

this required a switch to change the sync strategy based on rebar parameter.

However tests could be slow on macbook with OTP16 and sync - so timeouts added in unit tests, and ct tests sync_startegy changed to not sync for OTP16.

src/leveled_penciller.erl

@@ -1335,6 +1335,7 @@ add_missing_hash({K, {SQN, ST, MD}}) ->
 clean_dir_test() ->
     % Pointless gesture to test coverage
     RootPath = "../test/ledger",
+    ok = filelib:ensure_dir(RootPath),
     ?assertMatch(ok, file:write_file(RootPath ++ "/test.bob", "hello")),
     ok = clean_subdir(RootPath ++ "/test.bob"),
     ok = file:delete(RootPath ++ "/test.bob").