[perl #128283] [RFC] Disallow strings as ops in cmp-ok in Test.pm6

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# New Ticket Created by  Zoffix Znet 
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The operators for `cmp-ok` subroutine can be given as strings or as Callables. 
The problem with strings is the subroutine evals them as EVAL "&infix:<$op>" 
and the current implementation makes it impossible to use, say '<' as an 
operator:

    <Zoffix> m: use Test; cmp-ok 2, '<', 5, 'seems sane';
    <camelia> rakudo-moar beb3c9: OUTPUT«not ok 1 - seems sane␤␤# Failed test 
'seems sane'␤# at /tmp/kqui3siS7u line 1␤# Could not use '<' as a comparator␤»

It does work with &[<] instead of '<' and considering that's only 1 character 
longer and there's no 100% clean way to allow any op in a string, I propose we 
ditch the string version entirely.

While I don't know anything about SETTINGS, the current implementation also has 
a security hole, at least in camelia where `run`, while restricted in general 
code, is given a free pass when injected inside the string comparator passed to 
cmp-ok:

    <Zoffix> m: run "ls"
    <camelia> rakudo-moar beb3c9: OUTPUT«run is disallowed in restricted 
setting␤  in sub restricted at src/RESTRICTED.setting line 1␤  in sub run at 
src/RESTRICTED.setting line 14␤  in block <unit> at /tmp/wWcCMBi30n line 1␤␤»
    <Zoffix> m: use Test; cmp-ok '', '~~>;warn run "ls"; <z', '', '';
    <camelia> rakudo-moar beb3c9: 
OUTPUT«Perlito␤dalek-queue␤evalbot␤evalbot.log␤foo␤lib␤log␤mbox␤nqp-js␤p1␤p2␤p6eval-token␤perl5␤rakudo-j-1␤rakudo-j-2␤rakudo-j-inst␤rakudo-j-inst-1␤rakudo-j-inst-2␤rakudo-m-1␤rakudo-m-2␤rakudo-m-inst␤rakudo-m-inst-1␤rak…»