Support building with Test::More < 0.88

done_testing was introduced in Test::More 0.88; by avoiding its use
and specifying a test count instead, the whole test suite can be run
using older versions of Test::More (I've verified that it works with
Test::More 0.47 for instance).

Alternatively, the Test::More version requirement should be
specified in the package metadata.
Merge pull request #3 from pghmcfc/old_test_more

Support building with Test::More < 0.88