Add additional tests for results_df with PENDING trials

[bpkroth]

Per the discussions, can you please also add a test to check that pending trials that don't yet have results, still show up in the results_df, with empty results columns?

Originally posted by @bpkroth in #889 (comment)

Given the way that the _dummy_run_exp works in tests/storage/sql/fixtures.py this is probably easiest to do if we first add the API in the storage layer for enqueuing a new trial manually.
See Also: #687