Under cygwin 'PPI not installed' was issued when PPI was in fact
installed but Clone and/or List::MoreUtils were not and thus failed to
load from PPI. This is fixed by appending $@ to PPI_problem.
Also, PPI checking logic was refactored into Marpa::R2::Test to avoid
duplication. This required use'ing Marpa::R2::Test before BEGIN.
All changes in curly2.t after '-use Marpa::R2::Test;' are tabs to