Break up long tests

[bhazelton]

There are a number of really long tests in test_uvdata.py that really should be broken up, probably using the pytest's parametrize feature.

A non-exhaustive list identified in #967 includes: test_reorder_freqs test_sum_vis_errors test_flex_spw_add_concat test_break_add test_smart_slicing

[kartographer]

FWIW, I've gone through and cleaned up all but test_smart_slicing in #979, either because I was cleaning them up anyways (per a request from @plaplant to break up some monolithic tests), or because I was the one who originally wrote the ultra-long test (opps). But there are a few more that are fairly long -- choosing a fairly arbitrary cutoff of 150 lines of non-blank lines (excluding @pytest calls), where there's a bit of a break in the distribution of test lengths:

• test_add (339 lines; UVData version)
• test_select_with_ant_str (338 lines)
• test_fast_concat (334 lines)
• test_add (310 lines; UVBeam version)
• test_parse_ants (268 lines)
• test_add_drift (212 lines)
• test_miriad_location_handling (209 lines)
• test_conjugate_bls (205 lines)
• test_redundancy_contract_expand (200 lines)
• test_smart_slicing (193 lines)
• test_no_deg_units (175 lines)
• test_select_bls (162 lines)
• test_healpix_interpolation (153 lines)