Add a hyphen to the filename for wpt crashtests.

The convention is "firefox-bug-12345-crash.html" or
"chrome-bug-12345-crash.html" under wpt css folder.

Differential Revision: https://phabricator.services.mozilla.com/D214770

bugzilla-url: https://bugzilla.mozilla.org/show_bug.cgi?id=1904453
gecko-commit: ca2fc5d1ccfc56263eaa2b7142ca23fc371db2d3
gecko-reviewers: layout-reviewers, emilio

Author: aethanyc