[libstdc++ 1/2] Add testsuite comments, shuffle a bit of code.

[Benjamin Kosnik]

>Now that we have more experience with VERIFY and DEBUG_ASSERT, is there any
>reason to not have them on all the time?

No. Indeed, if you look through the testsuite, 

// { dg-options "-D_GLIBCXX_ASSERT -pthreads" { target *-*-solaris* } }

you'll see some testcases where this is done correctly, and others
(sadly including the last echristo/zack patch mentioned by Petur), where
it was not.

Really, -D_GLIBCXX_ASSERT should be on, by default, for all libstdc++
tests. That would suggest either --cxxflags in
scripts/testsuite_flags.in or DEFAULT_CXXFLAGS in
testsuite/lib/libstdc++.exp.

Since this is for libstdc++ only, I'm assuming it's DEFAULT_CXXFLAGS.
Sadly, dg-options kills CXXFLAGS.

-benjamin