[perl #45667] Apply tests from #119125 in reverse

Ticket #119125 represents the breakage caused by c30fc27b4df65a, which
fixed #45667 (/[#]$not_interpolated/x; a regression from 5.000) as a
side-effect of fixing /[#](?{this is not a code block})/x (a regres-
sion from 5.16.0).

It was backported to maint-5.18, which broke several modules, because
bug #45667 is *old*, and many modules depend on it.

So 02682386fe3e on the maint branch modified the fix to apply only to
code blocks, and added tests to make sure that #45667 wasn’t fixed.

#45667 is fixed in blead, but is untested, so grab the tests from
02682386fe3e and reverse the logic of the second one.