RT#86999 Fix blead dualvar regression

Since 5.19 it is pretty hard to create a PVNV dualvar in PP. Perl does not pollute a NV with a PV slot anymore
via do { my $x=3.0;my $y = "$x";$x }.
Improve the test to use explicit Scalar::Util::dualvar
Merge pull request #15 from rurban/519-dualvar

RT#86999 Fix blead dualvar regression