Add valid and invalid fixtures for legacy digital bags

[helrond]

Is your feature request related to a problem? Please describe.

In order to ensure that JSON files describing bags of legacy digital records are correctly validated, we need to add fixtures which represent valid and invalid instances.

Describe the solution you'd like

Add one or more JSON files to a legacy_digital_bag subdirectory in both the valid and invalid directories of the fixtures directory. So,

/rac_schemas/fixtures/valid/legacy_digital_bag
/rac_schemas/fixtures/invalid/legacy_digital_bag

Some things to test:

• the presence (or absence) of required fields
• valid (or invalid) controlled terms