Bug 1835066 - Tweak test_parser_diagnostics_unprintables.html to keep testing unexpected token errors. r=dholbert

Without nesting, the parser error is the unexpected token error, emitted
from cssparser.

With nesting enabled these behave the same as other strings outside of a
block (so an "unexpected end of input" error instead).

Tweak the test a bit to keep testing what this test wants to test (that
unexpected token errors are escaped).

Differential Revision: https://phabricator.services.mozilla.com/D179830