Test execution of examples in the CI

[pratikvn]

Currently we do not test examples within our CI. To make sure that all the examples work as expected, it would be nice to execute the examples and compare the outputs.

Current ideas:

1. In the documentation we already have expected output of the examples. On the CI we can execute the examples and perform a simple diff to compare the differences.
   1a. For examples, where non-deterministic behaviour is possible, we can also enforce checking within a tolerance.
2. Add a examples target to execute all examples (with make examples), with optional parameters to execute specific examples.

[tcojean]

For the make examples, we can maybe do something with default settings though some examples require explicit input (including from std::cin such as minimal-cuda-solver).
In addition, some examples in some cases will actually fail by design, again the minimal_cuda_example I believe will run only if we compile Ginkgo with GINKGO_BUILD_CUDA and a GPU device 0 is present.