fix Storable test for pre-5.19.2 threaded perls

fix Storable test for pre-5.19.2 threaded perls

The test of Storable's identity-preserving handling of the immortal
scalars ran into trouble with an old bug regarding overloading of the
PADTMP flag, which could prevent the test script getting actual references
to the immortal truth values in order to feed to Storable and to compare
its output against.  On the affected perls, compiling code that includes
a const op whose value is an immortal makes subsequent executions of the
refgen operator on that immortal scalar copy it, breaking its identity.
Loading the testing infrastructure modules could easily perform such
compilation, though whether it actually does depends on the versions of
the modules.  Work around this by taking the platinum-iridium immortal
references first thing in the test script, before loading anything that
could break it.