Python tests: Use compat date matching the enable date of the Python version

[hoodmane]

The goal here is to increase the real-world relevance of the tests. Before this
change, we test the oldest compat date and the newest compat date if we run
these variants but we don't test compat dates from the range that are most
likely to be used with each Python variant. This makes it so that each Python
test passes the enable date of the Python version it uses as the compat date.

I added a no_default_compat_date option to wd_test to remove the
--compat-date=... argument because we get the compat dates directly from
compatibility-date.capnp which can't flow into starlark so we can't pass it as
the compat_date argument to wd_test. Instead, I template the dates into the
wd-test file.

I took the minimum of the date in maximum-compatibility-date.txt and the
enable date to deal with flags with future enable dates. I also added an extra
--allow-future-compatibility-date flag to workerd since it seems that
maximum-compatibility-date.txt can contain future dates which workerd normally
rejects.

Only quite minor changes were needed to the actual tests. I separated the
template changes into a different commit because they are noisy.

cc @fhanau

[ask-bonk]

LGTM

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