psvi-required-001.xml should declare psvi-required

[ndw]

From private email:

While working with XProc test suite again, I am baffled by the
expected results of test "psvi-required-001.xml" over and over again.
Clearly an XProc implementation not supporting PSVI like Calumet or
mine cannot pass this test. But running the test it always
looks like I am producing wrong results on a supported step.

In my opinion reading the results of this test would be very much
clearer, if p:declare-step in the pipeline would use
attribute "psvi-required" so not supporting implementations can fail
with err:XD0022.

Would you agree or is there a reason to have the test in the current
version I did not get?

[ndw]

Agreed in the 20 Aug 2014 XProc WG meeting.