Tax fixes in unit test

Tax fixes in unit test

When this->isValidateFinancialsOnPostAssert is true the
test class checks that line items and payments are valid.

I'm trying to enable this for this class. However, there are some issues
that I have found fixes for (and at least 1 I'm still working on)
- some tests try to set tax_amount when it is not enabled
which is invalid - removed
- one test tries to use chaining in a way that
we know is not going to do a job of creating the entities
as it adds the payment before the line items. I switched
this to create a pending payment which doesn't alter the
thing under test & brings it closer to the
recommended flow
- one test is deliberately invalid - I marked it as
not eligible for the validation
- the price set id was not being passed to the Confirm->submit
function (accessed by tests, mostly via the ContributionPage.submit
api) - I added functionality to retrieve it