[perl #126700] [JVM] calling .index with negative start position on string does not return Nil

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# New Ticket Created by  Christian Bartolomaeus 
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rakudo.jvm seems to ignore that $pos is negative in the following example:

$ perl6-j -e 'say index("xxy", "y", -1)'
2

$ perl6-m -e 'say index("xxy", "y", -1)'
Nil

There are failing tests for rakudo.jvm in S32-str/index.t.