[2] Add tests to partner_request_spec.rb (can be split by 2-3 students)

[mdworken]

Please add specs ensuring the following to partner_request_spec.rb.

[1:2] Grouping 1:

• it disallows an item attribute if the quantity isn't set

• Tweak the title of make name required to properly indicate which field is actually being tested

[2:1] Grouping 2:

• it has a proof of partner status attachment

• it has a proof of form 990 attachment

• it can have one or more documents attached

[nihonjinrxs]

Per Sprint Planning Meeting, split into 3 issues, one for each group.