Move documentation about WALE_S3_INTEGRATION_TESTS

test_s3_worker is among the oldest of the tests and introduced the
WALE_S3_INTEGRATION_TESTS pattern.  Since then, it has enjoyed use in
other tests and its code has been factored out to another module, but
those tests do not document the convention.

So, instead move the advisory about that to that module and explain it
at a higher level.
Make WALE_S3_INTEGRATION_TESTS more sensitive

Instead of accepting any string as the affirmative, only accept
'TRUE'.

Previously, modifying WALE_S3_INTEGRATION_TESTS=FALSE to toggle
acceptance tests off would run them anyway, since 'FALSE' is an extant
string, which is all this used to care about.  That's confusing.