tests/integration: Use a simpler CMake test project

The 'step7' tutorial was fine, but it requires a C++ compiler to build.
I have replaced it with a simpler testcase which is based on the
existing amhello testcase.

This means that the integration test suite no longer requires a C++
compiler, and in practice this can allow us to avoid depending on the
GNU C/C++ compilers altogether and instead use the vastly smaller Tiny C
Compiler.