[tests] Infer the cxx_under_test (as clang++).

 - This is a reasonable default, and makes testing just work with no required
   parameters.

 - Add notes on all of the inferred or default values.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@174538 91177308-0d34-0410-b5e6-96231b3b80d8
Give a lot more timing latitude to some of the timing tests. Busy buildbots are hitting the timing limits too often.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@174539 91177308-0d34-0410-b5e6-96231b3b80d8