Introduces a new test API, `includesModule`. The method returns whether
a particular module like "ajax" or "deprecated" is included in the current
jQuery build; it handles the slim build as well. The util was created so that
we don't treat presence of particular APIs to decide whether to run a test as
then if we accidentally remove an API, the tests would still not fail.
Fixes gh-5069
Closes gh-5046
This commit fixes unit tests for the following builds:
1. The no-deprecated build: `custom:-deprecated`
2. The current slim build: `custom:-ajax,-effects`
3. The future (#4553) slim build: `custom:-ajax,-callbacks,-deferred,-effects`
It also adds separate Travis jobs for the no-deprecated & slim builds.
Closes gh-4577
