t/lib/common.pl can use caller to infer the name of the pragma under test.

t/lib/common.pl can use caller to infer the name of the pragma under test.

This avoids needing to use a variable in package main to pass in information.
Also, remove an unnecessary $ENV{PERL5LIB} assignment cargo-culted into most
users of t/lib/common,pl, and remove the BEGIN block as the code it contained
doesn't need to run at BEGIN time.