test: tie bce's terminfo declaration directly to its actual behavior
The declared-vs-omitted capability lists in this file are two
independently hand-maintained facts about the same code -- exactly
how bce drifted stale last time: this file asserted bce was *absent*
with a comment claiming erase ops always fill with COLOR_DEFAULT,
which quietly went wrong the moment blank_cell(g) started filling
with the active background instead, and nothing caught it until it
corrupted a real nvim session.
Adds a test that performs an actual erase with an active background,
checks whether the result behaves bce-style, and asserts the
declaration matches that behavior rather than checking them as
separate claims. Verified it fails when the two are put out of sync
(temporarily undeclaring bce against unchanged behavior) and passes
once they agree again.