TST: Cover resample bar first_trading_day method.
TST: Cover resample bar first_trading_day method.

Add a test to directly cover the first_trading_day method via the
`test_resample` suite. (The lack of coverage was exposed when testing
against real data.)

Also, refactor resample bar tests so that session bar reader is set up
in instance fixture.

Scapogo/zipline,quantopian/zipline,magne-max/zipline-ja,enigmampc/catalyst,florentchandelier/zipline,magne-max/zipline-ja,Scapogo/zipline,alphaBenj/zipline,humdings/zipline,alphaBenj/zipline,quantopian/zipline,bartosh/zipline,humdings/zipline,enigmampc/catalyst,bartosh/zipline,florentchandelier/zipline