[ci] Compile the produced code as part of the test suite

[vgvassilev]

Currently clad tests are organized in two ways. Clad can be attached to clang and it can produce an executable file which we run and/or it can produce source code which we verify with FileCheck.

The problem with verification statement by statement is that if we make a core change the changes in the output checks are painful.

To mitigate this problem we can implement a testing mode which pipes the produced source code to the compiler and if it compiles we can assume the test output is reasonable. We will still have a number of tests that make sure the code is still readable for humans but that should be more focused and won't need that much manual interventions.

[vgvassilev]

The problem with verification statement by statement is that if we make a core change the changes in the output checks are painful.

This part should be fixed by #875.