Prefer `expect` over `assert` for testing

Replaces `assert` with `expect` in tests.  This commit covers the following tests as specified on #2797:

- [x] acceptance
    - [x] addon-smoke-test-slow.js
    - [x] blueprint-test-slow.js
    - [x] brocfile-smoke-test-slow.js
    - [x] destroy-test.js
    - [x] express-server-restart-slow.js
    - [x] generate-test.js
    - [x] help-test.js
    - [x] init-test.js
    - [x] install-addon-test-slow.js
    - [x] new-test.js
    - [x] pods-destroy-test.js
    - [x] pods-generate-test.js
    - [x] smoke-test-slow.js
- [x] helpers
    - [x] assert-file-equals.js
    - [x] assert-dir-empty.js (extracted the same function from a few tests)
    - [x] assert-file.js