Porting/bench.pl: add --autolabel option

Porting/bench.pl: add --autolabel option

This automatically creates a unique label per perl interpreter.
For example

    Porting/bench.pl --autolabel \
        ~/bin/perl-5.20.0 ~/bin/perl-5.22.0 ~/bin/perl-5.24.0

is equivalent to

    Porting/bench.pl \
        ~/bin/perl-5.20.0=5.20.0 ~/bin/perl-5.22.0=5.22.0 \
        ~/bin/perl-5.24.0=5.24.0

As part of adding a test, this commit also slightly fixes up the test code
which generates a regex from an NNN.NN part of a template.