Audit use of system.indexes in jstests

Remaining occurrences of system.indexes outside of jstests/mmap_v1 are one of:
- comments
- auth tests
  - these might need another audit
- jstests/core/indexes_on_indexes.js
  - asserts you can't create indexes on system.indexes
  - seems like it should be true for any storage engine
    - Is it legal to create indexes on any system collection?