fix conflict between skip_all and END section

since the commit 430de781809a6be3bcd25a349dc40ce54405ab53
the test suite fails in cross-compil environment (perl-5.21.6 & perl-5.21.7)
like this :

    $ ./perl harness -v ../cpan/ExtUtils-MakeMaker/t/INSTALL_BASE.t
    ../cpan/ExtUtils-MakeMaker/t/INSTALL_BASE.t ..
    1..0 # SKIP cross-compiling and make not available
    ok 1 - chdir updir
    ok 2 - teardown
    # Looks like you planned 0 tests but ran 2.
    skipped: cross-compiling and make not available

this commit restores the implicit call of plan() at import time of Test::More

Signed-off-by: Francois Perrad <francois.perrad@gadz.org>