RFC: add a testsuite for libstdc++ pretty-printers

[Benjamin Kosnik]

> 	* testsuite/libstdc++-pp/simple.cc: New file.
> 	* testsuite/lib/gdb-test.exp: New file.
> 	* testsuite/libstdc++-pp/pp.exp: New file.

feel free to spell out prettyprinters instead of using pp, if you want
to make it very obvious what is going on in this directory.

we really strive for clarity given that this testsuite is so large

-benjamin