…sues: use iomgr backup poller when EE is disabled (v1.73.x backport) (#39950)

Backport of #39894 to v1.73.x.
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Fix the issue with gRPC Python Client not recovering in certain
situations: #38290, #39113, #39631.

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This PR:
- Keeps iomgr backup poller enabled on platforms that do not support
that have EventEngine / POSIX poller disabled.
- Fixes and hardens Python's reconnect_test.

This fixes issues with Python Client recovery in certain cases when it
falls back to the backup poller. Note that currently Python disables
POSIX poller manually via `GRPC_PLATFORM_SUPPORTS_POSIX_POLLING` due to
currently unresolved fork issue.

Unlike #39131, this change is meant to be tested and merged to master,
and then backported to past branches with broken python client reconnect
logic.

However, once Event Engine fork support is fixed, Python will switch to
it by removing `GRPC_DO_NOT_INSTANTIATE_POSIX_POLLER` from all builds.
After that, the backup poller should not be needed for Python. With the
updated test, the case with python relying on the backup poller is
covered, and we should be able to detect issues with Python having to
rely on in after switching to EE.

Co-authored-by: Sergii Tkachenko <sergiitk@google.com>

Change was created by the release automation script. See
go/grpc-release.