Allow test runs with zero assertions to pass

[jayjayjpg]

Fixes #1405

Previous Behaviour

Before this change, test runs that included tests which had in total zero assertions, would fail with a global error. This made it, f.e. not possible to run test cases leveraging other means of assertions than the QUnit built-ins (assert.ok, assert.equals, etc.)
For a reproduction example, see also the description of the original issue: #1405

New Behaviour

With this change, QUnit will decide if the current test run is "empty" and should therefore fail based on the number of tests included in the run rather than the number of included assertions. This should allow users to run test sets containing zero assertions, as long as respective test cases are marked as such using the assert.expect(0) declaration.

How to Test

Run the new example test case containing no assertions to verify the new behaviourL

bin/qunit.js test/cli/fixtures/zero-assertions.js

before	after

[jsf-clabot]

All committers have signed the CLA.

[Krinkle]

Look pretty good. One potential issue I found, otherwise should land shortly :)

[jayjayjpg]

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