feat(ngMock): add `$flushPendingTasks()` and `$verifyNoPendingTasks()`

`$flushPendingTasks([delay])` allows flushing all pending tasks (or up
to a specific delay). This includes `$timeout`s, `$q` promises and tasks
scheduled via `$rootScope.$applyAsync()` and `$rootScope.$evalAsync()`.
(ATM, it only flushes tasks scheduled via `$browser.defer()`, which does
not include `$http` requests and `$route` transitions.)

`$verifyNoPendingTasks([taskType])` allows verifying that there are no
pending tasks (in general or of a specific type). This includes tasks
flushed by `$flushPendingTasks()` as well as pending `$http` requests
and in-progress `$route` transitions.

Background:
`ngMock/$timeout` has `flush()` and `verifyNoPendingTasks()` methods,
but they take all kinds of tasks into account which is confusing. For
example, `$timeout.verifyNoPendingTasks()` can fail (even if there are
no pending timeouts) because of an unrelated pending `$http` request.

This behavior is retained for backwards compatibility, but the new
methods are more generic (and thus less confusing) and also allow
more fine-grained control (when appropriate).

Closes #14336