gh-152190: Fix memory checking failure in `test_zipfile64.py` (GH-152203)

* gh-152190: Fix memory checking failure in `test_strip_removed_large_file_with_dd_no_sig`

Remove the overly restrictive `allowed_memory` override (200 KiB) in
`test_strip_removed_large_file_with_dd_no_sig` to prevent a memory
checking failure.

* gh-152190: Revise comment about the empirical memory threshold

* gh-152190: Improve memory checking accuracy for `test_zipfile64`

Introduce the `assert_memory_usage` context manager to narrow the scope
of tracemalloc tracking down exclusively to the file-repacking phase.

This prevents previous file-writing side effects from interfering with
the baseline, improves tracing accuracy, and eliminates redundant inner
sub-function wrappers.

* gh-152190: Improve coding style and docstrings

* gh-152190: Remove unneeded comments and checks

Remove redundant "TESTFN2" disk space warnings from TestRepack, as
these tests exclusively use TemporaryFile().

Additionally, remove the repetitive `self.assertFalse(f.closed)` checks
from `TestRepack` methods since it's already verified in
`TestsWithSourceFile`.

* gh-152190: Further optimize tests and tidy code

Rename `TestRepack` to `TestRepacker` to better reflect its coverage of
`zipfile._Repacker`.

Move heavy text chunk generation and common constants from `setUp`
to `setUpClass` to ensure single initialization across tests.

Clean up repetitive local definitions of filenames and lorem text by
promoting them to class properties.

Reduce the test file size from 8 GiB to 4.1 GiB, which is large enough
to trigger ZIP64 extension and sufficient for the testing purpose.

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Co-authored-by: Zachary Ware <zach@python.org>