qunit: Make eslint config pass on qunit test files

qunit: Make eslint config pass on qunit test files

Follows-up c0fb8a8836, I890e6e49b.

* Disable 'qunit' env in general source code. And re-declare
  locally in the few src files that use it properly.

* Create separate eslint config for tests/qunit with various
  rules disabled (e.g. valid-jsdoc and es3-keywords).

Change-Id: I37ccec2019de55edfee92697eb80478df7cb6220