Add dist test or utility function

[agjohnson]

Distribution/OS checking could be built into a utility function or a proper test. It could be used like:

my $t = dist(
    ubuntu => noop(1),
    centos => noop(0),
    netbsd => noop(0)
);

default return would be undef -- this will require processing change. Pkg test could drop the hash parsing, but keep distribution checking for finding which package manager to use.