overhaul dist/B-Deparse/t/core.t

overhaul dist/B-Deparse/t/core.t

Originally, this test file just checked that CORE::foo got correctly
deparsed as CORE::foo, hence the name. This commit expands it
to fully test both CORE:: verses none, plus that any arguments
are correctly deparsed. It tests many more keywords, and it also
cross-checks against regen/keywords.pl to make sure we've tested all
keywords, and with the correct strength.

(There is very little of the original file left.)