chore(travis): split unit test jobs into 'core' and 'jquery'

"unit-core" consists of code+jqlite, module test, and promise A+ tests.
"unit-jquery" is code+jquery
"docs-app" includes unit and e2e tests

Splitting the unit tests into more than one job makes it faster
to rerun jobs that fail because Safari or Edge cannot complete the
suite, which seemingly happens on random.

Closes #16292