Simplify Makefile.PL requirements

Requirements accumulate from one phase to the next ('configure' -> 'build'
(which also requires 'runtime' requirements) -> 'test') so there is no need
for 'test' to list requirements already listed for 'configure', 'build' or
'runtime' (or for 'build' to list requirements already listed for
'configure' or 'runtime').

(However, 'runtime' must still list everything required after installation,
but does not need to list 'perl' since 'runtime' is the phase that
'MIN_PERL_VERSION' is applied to anyway.)