Revert "Move declaration of default TS_DIFF to top of file"

This reverts commit 6a1ec180242c23658b5311f8e5cd1a932c4b92a6.

Turns out this results in an unbound variable error - TS_DIFF needs to be
exported very early on, or alternatively, a default needs to be set at the
point of use.
Add note about testing background jobs

I debated making bg job cleanup a default for teardown.  Decided against it
because a) it is easily solved and b) it messes with the idea that teardown
does nothing by default.  There is no super in bash to make it easy to do the
default teardown so I feel it's important that it does nothing - that way users
will not accidentally miss out on some expected behavior.