Avoid use_ok(); it can cause weird runtime errors after a compilation failure

Since use_ok() catches normally fatal compilation failures when loading
a module, it appears the process may be left with a half-compiled module
loaded which can be partially used, causing extremely odd error cases.

In the case of t/tofasta.t, failure of the tofasta operation to load
cleanly due to missing Bio::SeqIO caused subsequent ->test_output()
calls to successfully create tofasta objects (!) but then enter infinite
recursion.  I'm assuming the recursion happened because the full tofasta
package wasn't loaded, but it still inherited methods from the base
class.

Just switch to normal "use"; if it doesn't work, the test will fail
anyway.

Thanks to Alexandr Ciornii <alexchorny@gmail.com> for the initial report
about t/tofasta.t hanging in infinite recursion on prereq failure.
0.13