[libcxx] [test] Check for C++ headers before building a test that uses them

When the libcxx test framework is executed within libunwind, there
are no standard C++ headers available (libunwind builds with
-nostdinc++, but doesn't add any libcxx headers to the include path).

Check that a test that includes <iostream> can be compiled before trying
to build and execute a test program that includes it.

Previously, the compile error here would block all libunwind tests from
executing altogether.

Differential Revision: https://reviews.llvm.org/D147630