[grpclb and fake resolver] clean up e2e tests and simplify fake resolver (#34887)

Changes to fake resolver:
- Add `WaitForReresolutionRequest()` method to fake resolver response
generator to allow tests to tell when re-resolution has been requested.
- Change fake resolver response generator API to have only one mechanism
for injecting results, regardless of whether the result is an error or
whether it's triggered by a re-resolution.

Changes to grpclb_end2end_test:
- Change balancer interface such that instead of setting a list of
responses with fixed delays, the test can control exactly when each
response is set.
- Change balancer impl to always send the initial LB response, as
expected by the grpclb protocol.
- Change balancer impl to always read load reports, even if load
reporting is not expected to be enabled. (The latter case will still
cause the test to fail.) Reads are done in a different thread than
writes.
- Allow each test to directly control how many backends and balancers
are started and the client load reporting interval, so that (a) we don't
waste resources starting servers we don't need and (b) there is no need
to arbitrarily split tests across different test classes.
- Add timeouts to `WaitForAllBackends()` functionality, so that tests
will fail with a useful error rather than timing out.
- Improved ergonomics of various helper functions in the test framework.

In the process of making these changes, I found a couple of bugs:
- A bug in pick_first, which I fixed in #34885.
- A bug in grpclb, in which we were using the wrong condition to decide
whether to propagate a re-resolution request from the child policy,
which I've fixed in this PR. (This bug probably originated way back in
#18344.)

This should address a lot of the flakes seen in grpclb_e2e_test
recently.