Revert several changes that broke tests
This is based on #15158 by @aneeshusa, with additional reverts.
This reverts #15064, which is causing many tests not to run, and #15129 and #15155 which landed while tests were not running and may have caused some new failures in iframe tests.
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Also remove the obsolete `--cpu` and `--gpu` renderer flags,
which also are no longer used.
Update tests and wptrunner to not pass these flags.
wptrunner changes have been upstreamed in:
- w3c/wptrunner#224
- w3c/wptrunner#226
Index the session past correctly when discarding.
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Oops, indexed from the wrong end when discarding documents in #14312.
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- [X] `./mach test-tidy` does not report any errors
- [X] These changes do not require tests because we're not testing document discarding
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