Better support for join operators in test builder

[agouy]

Error while building test for https://github.com/tercen/faust_operator with the faust_example.csv dummy dataset.

> join_res = df_out %>%
+   ctx$addNamespace() %>%
+   as_relation() %>%
+   left_join_relation(ctx$crelation, ".i", ctx$crelation$rids) %>%
+   left_join_relation(join_png, list(), list()) %>%
+   as_join_operator(ctx$cnames, ctx$cnames)
> tim::build_test_data(join_res, ctx, "test_1", relTol = 1e-5)
Error in env$toTson() : attempt to apply non-function