Improve manual display tests of undisplayable chars (bug#58168)

Improve manual display tests of undisplayable chars (bug#58168)

Test display of multibyte raw bytes, as well as undisplayable
multibyte chars (C1 controls and other values).

The test still assumes that raw bytes should be displayed identically
to undisplayable characters (such as C1 controls) because that is how
the display code currently works.

* test/manual/redisplay-testsuite.el (test-redisplay-5-toggle)
(test-redisplay-5): Fix likely typo (#x3fffc) of raw byte value.
(test-redisplay-6): New.